As soldiers mopped up pockets of resistance and the government declared it was back in control, fears grew Thursday that the tentative quiet restored to Thailand's capital after a bloody crackdown on protests may just be a respite from violence and political polarization that could continue for years. 
 
 Leaders of the anti-government Red Shirt movement vowed a return as they were taken into custody. 
 
 "I think this is a new beginning for the Red Shirts," said Kevin Hewison, a Thailand expert at the University of North Carolina. "It will be a darker and grimmer time of struggle and less-focused activities. By no stretch of the imagination is the movement finished." 
 
 The Thai government declared Thursday it had mostly quelled 10 weeks of violent protests in the capital as buildings smoldered, troops rooted out die-hard holdouts and some residents cautiously attempted a return to normal life a day after a military operation cleared the main commercial district of thousands of demonstrators, leaving 15 dead and nearly 100 injured. 
 
 Troops roamed the city on foot and in Humvees and exchanged gunfire with scattered Red Shirt holdouts, who fought near the city's Victory Monument and torched a bank, bringing to 40 the number of buildings set aflame after the military push sent the protesters retreating from their demonstration site. 
 
 The protesters, demanding elections, had fortified themselves behind tire-and-bamboo-spike barricades. 
 
 Thailand's finance ministry estimated the economic damage to the country at 50 billion baht ($1.5 billion). Continued security concerns led officials to extend a nighttime curfew in Bangkok and 23 other provinces for three more days. 
 
 Even so, army spokesman Col. Sansern Kaewkamnerd said the government was in charge. 
 
 "Overall, we have the situation under control," he said. 
 
 Residents, meanwhile, moved carefully to resume their routines. 
 
 Cleaning up 
 
 With military checkpoints closing, city workers removed debris and collected piles of garbage left in the streets. Residents in protest areas were able to leave home to shop. Electricity was restored in many areas. 
 
 But many of those who ventured into the streets were still deeply shaken by the violence. Video: Protesters burn buildings in Bangkok 
 
 "This really worries me — this shouldn't happen to Thailand," said Somjit Suksumrain, a construction company manager. "Thailand should not end up like this." 
 
 By late Thursday, authorities had taken into custody most of the senior Red Shirt leaders. 
 
 Three surrendered Thursday after five others gave themselves up the previous day and were flown to a military camp south of Bangkok for interrogation. 
 
 "I'd like to ask all sides to calm down and talk with each other in a peaceful manner," Veera Musikapong said after being taken into custody Thursday. "We cannot create democracy with anger." 
 
 Not all were as conciliatory. 
 
 Somyot Pruksakasemsuk, another Red Shirt leader, said the movement was simply regrouping. 
 
 "Initially, independent movements of the masses in Bangkok and the regions will begin, then riots will ensue," he said. "For Thailand in the long term, there will be major changes due to the crisis of faith." 
 
 Demand for new elections 
 
 The government described the mayhem as organized terrorism. Officials also said the arson and looting after the troops quashed the main protest were "anticipated aftershocks" that did not represent deeper trouble. 
 
 Still, government spokesman Panithan Wattanayagorn acknowledged the protesters had sympathizers among the broader populace, and said the rioting was sparked by disappointment, hopelessness and anger. But he said it was only as large as it was because of "prior organized planning." 
 
 The Red Shirts had demanded the ouster of Prime Minister Abhisit Vejjajiva's government and new elections. The protesters, many of them poor farmers or members of the urban underclass, say Abhisit came to power illegitimately and is oblivious to their plight. 
 
 Analysts said Abhisit was under increased pressure to hold early elections. 
 
 "Abhisit still has to hold elections by next year, and he could be under pressure still to do it earlier, by his original November offer," said Paul Handley, the author of a biography of Thailand's king. "Thailand still needs to hit this reset button. ... Even if Abhisit's government is technically legal, the Reds' widespread perception that it is illegitimate remains." 
 
 The crackdown should silence the large number of government supporters who were urging a harder line, and the rioting that followed may extinguish some of the widespread sympathy for the protesters' cause. ||||| Reuters Firefighters spray water at the Central World shopping mall building set on fire by anti government "red shirt" protesters in Bangkok May 20, 2010. 
 
 BANGKOK—Thick columns of smoke hung over some of Bangkok's priciest real estate early Thursday while security forces intermittently fired at militant antigovernment protesters still hunkered down in pockets of the city, stoking fears that Thailand's chronic political divides could degenerate into an even-deeper conflict. 
 
 Demonstrators and local street thugs started fires after an army assault on the so-called Red Shirt protesters' main camp Wednesday morning forced the group's top leaders to surrender to police to prevent massive bloodletting. Mobs of roving protesters singled out economic targets for special attention, setting Thailand's stock-exchange headquarters ablaze and torching several banks and the headquarters of the city's electricity provider. Large parts of the Central World shopping mall overlooking the demonstrators' camp in Bangkok's main commercial corridor were gutted by flames and some sections collapsed as other malls smouldered nearby. 
 
 Thailand's government declared a curfew—from 8 p.m. to 6 a.m.—for the first time since pro-democracy protests in 1992, and hospital officials say five protesters and an Italian photojournalist were killed. The Associated Press reported witnesses as saying at least six other bodies were found in a Buddhist temple inside the protest area, which had been a sanctuary from the violence, but later became the site of violent clashes that continued into Thursday morning. 
 
 Urban Chaos View Slideshow European Pressphoto Agency A Bangkok resident peered out as soldiers stood amidst the mess of tires and other litter left from street fighting. 
 
 Battles between militant protesters and soldiers continued Wednesday night, turning gritty working-class areas in central Bangkok more dangerous. Prime Minister Abhisit Vejjajiva said in a televised address Wednesday that order would soon be restored. 
 
 "Please rest assured that the government, officers and I are confident and determined that we can overcome the problems," Mr. Abhisit said. 
 
 Government officials said they had expected a violent backlash when they moved to shut down the monthslong antigovernment protest. Satit Wongnongtaey, a minister in the prime minister's office, told a news conference that the government and security forces had anticipated "aftershocks" from the more-militant fringes of the Red Shirt movement. 
 
 Political analysts, though, worried that these aftershocks have worsened a struggle pitting Thailand's conservative establishment against a rural, populist movement inspired by ousted prime minister and telecommunications billionaire Thaksin Shinawatra, who lives in self-imposed exile to avoid imprisonment on a corruption conviction, but supports the movement financially from abroad. 
 
 Some said Wednesday's crackdown and the violent reprisals pose a fresh risk to the political survival of Mr. Abhisit's government, which has withdrawn an offer to hold elections in November. Protest leaders had demanded immediate polls to reset Thailand's political system, which they say is manipulated by powerful bureaucrats and army leaders. Their failure to secure their goal is adding to their anger. 
 
 "The list of episodes that embitter people on the Red side has just been lengthened," said Michael Montesano, an expert on Thailand and a visiting research fellow at the Institute of Southeast Asian Studies in Singapore. 
 
 At the same time, the Red Shirts may be capable of outflanking the armed forces with hit-and-run attacks. "The Reds feel they've been disenfranchised, and now they don't respect the system anymore," said Thitinan Pongsudhirak, a professor of political science at Bangkok's Chulalongkorn University. He says Mr. Abhisit's government is still struggling to find a way to peacefully resolve the country's conflicts. 
 
 Outside Bangkok, protesters in the northern city of Chiang Mai built barricades around government buildings while demonstrators set fire to government offices in Udon Thani and damaged the City Hall in Khon Kaen, both in northeastern Thailand. Later Wednesday, the Thai government expanded the Bangkok curfew to cover 23 of Thailand's 76 provinces. 
 
 Economists said the rioting could further damage Thailand's economic prospects at a time when its peers are rebounding sharply from the global financial slump. In the worst-case scenario, violent protests could spread to other cities, destabilizing the country in a way that could alter perceptions about this open, export-driven economy for years. Sanjay Mathur, Asia economist at the Royal Bank of Scotland in Singapore, says he thinks Thailand could take a hit if the Bangkok rioting is replicated widely in other parts of the country. 
 
 In a report released after the protest leaders surrendered, Goldman Sachs economists warned that Thailand still faces a number of major hurdles before investors can be assured it is back on track, including the need to call new elections and pass measures to reduce the country's widening income disparity. Such overhauls require a stable political environment, the economists said, and "we believe the recent political events have pushed us further away from achieving that." Continuing uncertainties in the country's political climate "could put Thailand in a more disadvantaged position compared to its regional peers down the road." 
 
 Thai Red Shirt protest leaders called off their rally and surrendered after an army assault on their camp in Bangkok, but hardliners turned the city into a war zone, attacking security forces and setting fires. Patrick Barta discusses. 
 
 William Heinecke, chairman and chief executive of hotel and retailing company Minor International PCL, told The Wall Street Journal that his firm can quickly reopen hotels and restaurants, but worried that tourists might give Thailand a wide berth for some time. "We can be up and operating very quickly, but whether there will be people is another question," Mr. Heinecke said. 
 
 Some investors and executives, though, see the clearing of the main demonstration in central Bangkok as an indication that Thailand is getting back to some kind of normality. 
 
 "There is still uncertainty, but we think the downside is fairly limited from here," said Grace Tam, vice president of investment services at J.P. Morgan Asset Management in Hong Kong. Local businessman Peter van Haren, chief executive of Thailand-based construction-materials firm Wiik-Hoeglund and former chairman of the Joint Foreign Chambers of Commerce in Thailand, says that while the violence has hurt the country's image in the short run, the Thai authorities have also shown they are able to enforce the law. 
 
 "How laws are enforced or upheld is a good indicator of your ability to be able to uphold your investment," Mr. van Haren says. 
 
 Bangkok's marathon street protests started peacefully. On March 12, tens of thousands of demonstrators began flowing into the Thai capital to push their demand for new elections. Parliament elected Mr. Abhisit as prime minister in 2008 after the armed forces brokered a series ofdefections from a pro-Thaksin party in Thailand's legislature. 
 
 As the protests dragged on, however, a violent fringe group began to dominate the rallies. They used the fortified perimeter around the Red Shirts' main protest site in central Bangkok to shoot fireworks and other improvised explosives at security forces sent to contain the protesters after they dawdled in accepting Mr. Abhisit's offer earlier this month of new elections on Nov. 14. 
 
 Senior government leaders such as Finance Minister Korn Chatikavanij, as well as government negotiators and some opposition mediators, blame Mr. Thaksin for the collapse in talks. They say he encouraged Red Shirt leaders to delay providing a response to Mr. Abhisit's offer, eventually leaving the prime minister no option but to send in troops. Mr. Thaksin denies sabotaging the peace talks. 
 
 Then, at dawn Wednesday, hundreds of soldiers began massing on the approach roads to the heavily fortified protest camp. Midmorning, armored tanks rammed into barriers constructed from sharpened bamboo staves and kerosene-soaked tires, and soldiers slowly ventured into the camp as protest leaders prepared to surrender and hard-liners fired grenades. 
 
 Speaking at the protesters' main stage in the early afternoon, one of their leaders, Jatuporn Prompan, explained they had to call off the rally to prevent any more people dying. At least 74 people have been killed since the demonstrators launched their campaign, with more than half of those coming in recent days. 
 
 "We know this decision will pain you," said Mr. Jatuporn, wearing a white T-shirt bearing the image of Indian protest leader Mohandas Gandhi. "But we have to stop the death, even though our fight will carry on." 
 
 As he spoke, explosions could be heard and militant Red Shirts in the area began setting fires, filling the afternoon air with choking black smoke. People in the crowd, many of them women and elderly, wept and pleaded for Mr. Jatuporn and another leader, Nattawut Saikua, to change their minds. Instead, the two men went with other Red Shirts to surrender at Bangkok's police headquarters nearby, and Thailand's crisis entered a dangerous and unpredictable new phase. 
 
 —Eric Bellman, Patrick Barta, Wilawan Watcharasakwet and Alex Frangos contributed to this article. 
 
 AFP/Getty Images Hardliners set the Thai stock exchange building in Bangkok on fire even though leaders of the mainstream "Red Shirt" protesters surrendered to the army. 
 
 Write to James Hookway at james.hookway@wsj.com
– Seven Thai protest leaders surrendered in Bangkok today and called off their uprising, but Bangkok continued to erupt in violence as Red Shirts torched buildings, including the Thai stock exchange, a TV station, a utility company, and shopping malls. Leaders surrendered after troops stormed a protest encampment, killing at least 5, including an Italian news photographer. A military commander called the situation "under control," as explosions continued and smoke engulfed the commercial center of Bangkok, reports the Wall Street Journal. The unrest spread to northeast Thailand, where protesters stormed a town hall complex in the city of Udon Thani, reports MSNBC. The 7 men, called "terrorist leaders" by government officials, said they were surrendering to end the bloodshed, reports AP. "I apologize to you all but I don't want any more losses. I am devastated," leader Jatuporn Prompan told protesters shortly before his arrest. "We will surrender."