A standoff between police and anti-war demonstrators near the NATO summit at McCormick Place left several protesters bloodied and led to dozens of arrests, as delegates gathered for the first of two days of meetings.The confrontation, which flared up late this afternoon near the corner of Michigan Avenue and Cermak Road, lasted for almost two hours, as officers in full riot gear went nose to nose with demonstrators, with police determined to keep the crowd away from the convention center where NATO delegates were meeting.It was not immediately clear how many people were injured in the tussle, but by 7 p.m., seven protesters had been treated at Mercy Hospital and Medical Center, hospital spokeswoman Connie Murphy said.By 9 p.m. there had been at least 45 arrests, authorities said. There were four police officers with minor injuries, authorities said.Although officials were able to clear the McCormick Place area, as darkness fell, rogue groups of protesters flared up across the Loop, keeping police scrambling.Police seemed intent on keeping the ever-morphing packs of demonstrators south of the Chicago River. Police buses lined up along the Michigan Avenue Bridge, as large numbers of officers spread out through the area.At Van Buren and Wabash streets, police attempted to block a group of more than 100 protesters from heading north on Wabash. The protesters ran through their barricade, some getting into scuffles with police, who detained several protesters.The rest of the group kept moving.As one group of protesters marched south on Michigan, several hundred officers rushed to guard the doors at the Art Institute, where First Lady Michelle Obama was scheduled to host a dinner earlier this evening.Most of the protesters passed the museum and continued walking south, but some stayed behind, saying they wanted to "meet the Obamas."The evening activity followed a pitched battle between an overwhelming number of police and several hundred protesters outside McCormick Place. The skirmish broke out shortly after 4 p.m., at the end of a day-long protest that started in Grant Park and ended at a makeshift stage at Michigan and Cermak."NATO'S East! NATO's East!" the protesters chanted as they moved closer to the makeshift stage where rally leaders had spoken.A rally organizer told protesters that they had a street permit, but it had expired. He urged the crowd to leave."The police are going to move us off the street. We've been telling you this would happen," the organizer told to the crowd.Activist Andy Thayer used a bullhorn to implore the crowd to move."We went to a lot of trouble to make sure this march was a safe and peaceful march," he shouted.Police made announcements in both Spanish and English warning they might use water or sound or other anti-riot measures to disperse the crowd."You are hereby ordered to disperse," police said. "If you fail to comply you may be arrested or be subjected to other police action."Some protesters chanted "this is a peaceful protest," but others threw plastic bottles at police. ||||| Protesters clad in black clashed with police at the end of what had been a peaceful march and rally by thousands of demonstrators, led by disenchanted veterans of the Iraq and Afghan wars protesting the opening of the two-day NATO summit in Chicago today. 
 
 The demonstration was the largest the city has seen in years. 
 
 "It's crazy. There's so many people here," Esther Westlake, a recent graduate of Northeastern Illinois University, told The Associated Press. "Having NATO in town is kind of exciting." 
 
 The battle between protesters believed to be members of the anarchist group Black Bloc and police left several demonstrators bloodied, and marred what had been a solemn and orderly march. 
 
 At the end of the march, the vets threw their NATO medals over the fence set up by the Secret Service around McCormick Place. 
 
 Some of the veterans told ABC station WLS-TV in Chicago that they had hoped a NATO representative would meet the group and take the medals back as a symbol of recognition. 
 
 Former Chicago Police Superintendent Jody Weis criticized the black-clad demonstrators who clashed with police for undermining the emotional power of the veterans' act. 
 
 "You have classic Black Bloc ideology, peaceful ceremony, moving ceremony and these individuals use this as an occasion to disrupt, engage the police, engage in criminal activity," Weis told WLS-TV. "Once they crossed that behind and are throwing bribes at the police officers and hitting them with sticks and weapons, then they have no option but to maintain control. It is classic Black Bloc ideology. It ruins ceremonies and ruins a ceremony of veterans turning in their valor medals." 
 
 The demonstrators had a wide range of agendas; there were anti-war activists, people concerned about inaction on climate change, and people protesting the handling of the global economy. 
 
 But the activists on the street weren't the only ones aiming to disrupt the summit. A hacking group affiliated with Anonymous took responsibility for temporarily crippling the Chicago Police and NATO websites today. 
 
 Chicago police are working with federal authorities to investigate the attack and the extent of it, the Chicago Tribune reported. 
 
 NATO has not confirmed it was the victim of a cyber attack. All three sites now appear to be running as usual. 
 
 A lengthy statement from the hacking group, which called itself antis3curityops, was posted on Cyber War News, declaring: "We are in your harbor Chicago, and you will not forget us." 
 
 A Twitter user affiliated with Anonymous tweeted "Tango Down" with a link to the Chicago Police Department's website. "Reason: for violation of #humanrights," @Anon_Central wrote. 
 
 The attack was orchestrated using DDoS, a method in which numerous systems attack a single target website until it is forced to shut down. 
 
 Cole Stryker, author of "Epic Win For Anonymous," said today's hacking was likely more embarrassing than harmful to the Chicago Police Department. 
 
 "It's an egg on the face type situation," he said. "It's embarassing when the people who are supposed to keep you safe are so easily victimized by a prankish attack like this, however I don't think it's concerning them." 
 
 The internal systems at the city and police department would not have been affected by the attack, Stryker said. 
 
 The online disruptions were just the latest incidents in the chaos that has erupted around the two-day NATO summit of world leaders in Chicago. 
 
 Three men were arrested on terrorism charges Saturday. They are accused of building Molotov cocktails and planning attacks at President Obama's Chicago campaign headquarters and at the home of Chicago Mayor Rahm Emanuel during the NATO Summit, prosecutors said. 
 
 Brian Church, 20, of Ft. Lauderdale, Fla., 24-year-old Vincent Betterly of Oakland Park, Fla., and 24-year-old Jared Chase of Keene, N.H., are charged with conspiracy to commit terrorism, providing support for terrorism and possession of an explosive or incendiary device. 
 
 "These men were here to hurt people," Cook County State's Attorney Anita Alvarez said in a news conference. 
 
 The defendants are self-proclaimed members of the Black Bloc group. 
 
 In addition to materials to make molotov cocktails, police say the defendants had various weapons, including a mortar gun, swords, a hunting bow, throwing stars, knives, brass knucles. 
 
 "This plot does not represent protest behavior, this is criminal behavior," said Chicago Police Superintendent Garry Mccarthy. 
 
 The men argue the materials police collected in an overnight raid Wednesday were used to brew beer. 
 
 In a differant case, two militia men from Wisconsin were arrested after police found shotguns, shells, extended clips, knives and batons inside their vehicle during a traffic stop. Both men were wearing militia uniforms and were riding in a car that flew a militia flag, police said. 
 
 Police said they believe the men were in town to protest NATO. 
 
 A Chicago man was also charged with conspiring to build a Molotov cocktail and will appear in court today, the Sun-Times reported. 
 
 ABC News' Alex Perez and Andy Fies contributed to this report
– What began as an orderly march by thousands of activists protesting the NATO summit yesterday turned bloody as clashed erupted with Chicago police. At least seven people required hospital treatment, and dozens were arrested, reports the Chicago Tribune. The confrontation lasted close to two hours, and left several protesters bloodied, said the newspaper. In other action, a number of Iraq and Afghanistan war veterans threw their NATO valor medals over a fence set up by the Secret Service around the summit site at the McCormick convention center, reports ABC News. A hacking group affiliated with Anonymous also reportedly took down NATO and Chicago police websites temporarily. "We are in your harbor Chicago, and you will not forget us," said the group, antis3curityops, in a comment posted on Cyber War News. The demonstrations yesterday followed earlier arrests of five men on terrorism charges. Three were accused of building Molotov cocktails and planning attacks on President Obama's local campaign headquarters and on Mayor Rahm Emanuel's home during the summit. Two militia members from Wisconsin were also busted after police found shotguns, shells, knives and batons inside their vehicle. Police are steeling for more trouble today.