Moscow ( CNN ) -- Thousands of people in Moscow rallied for and against Vladimir Putin in separate rallies Monday after official election results showed the Russian prime minister handily winning back the presidency . International observers blasted the Sunday election , saying the outcome was never in doubt . Some foreign governments pledged to work with the new leader despite concerns about electoral violations . `` The election has not been exemplary , to say the least , '' said French Foreign Minister Alain Juppe . With more than 99 % of the votes counted , Putin received 63.75 % of the vote , easily avoiding a runoff in a field of five candidates . Putin 's closest challenger , Communist leader Gennady Zyuganov , got slightly more than 17 % , and the other three candidates -- including Mikhail Prokhorov , the owner of the New Jersey Nets basketball team -- were in the single digits . The result was still considered preliminary , however ; the state-run RIA Novosti news agency reported final results may come next week . Putin , 59 , declared victory before thousands of cheering supporters near Red Square on Sunday night , and a similar crowd returned Monday to celebrate his victory , RIA Novosti reported . The news agency said about 10,000 people were at Manezhnaya Square , where sports figures and celebrities congratulated Putin and denounced the opposition 's claims that the voting was unfair and rigged . Other pro-Putin rallies took place around the country , RIA Novosti said . Thousands of Putin opponents also turned out in central Moscow on Monday , and police arrested 250 people . Among them were opposition activists Ilya Yashin , Alexei Navalny and Sergey Udaltsov , the Moscow branch of the Russian Interior Ministry reported . Police detained 16 nationalists in central Moscow because they were attempting to create unrest , the ministry said , and roughly 50 supporters of the banned National Boshevik Party were detained for attempting to hold an unauthorized rally . `` All those arrested were taken to a police station for their actions to be processed , and for the authorities to decide whether to press charges , '' the Moscow branch of the ministry said . International observers blasted Russia 's presidential election Monday and expressed disappointment and frustration with the way Putin won . `` The point of an election is that the outcome should be uncertain . This was not the case in Russia , '' said Tonino Picula , the head of an observer mission from the Organization for Security and Cooperation in Europe ( OSCE ) . Observers said they observed ballot stuffing and other irregularities in about a third of polling stations they monitored , and an uneven playing field in the run-up to the election . Sounding somewhat exasperated , the Council of Europe 's Tiny Kox urged Russia `` to have a fair election , '' saying `` it 's not that difficult . '' Ambassador Heidi Tagliavini of the OSCE was particularly critical of irregularities in vote counting because `` what matters in an election is the counting , '' she said . She declined to say whether the irregularities affected the outcome of the vote -- a landslide for Putin by international standards , if not Russian ones . And she praised incremental improvements such as web cameras in polling stations and transparent ballot boxes , as well as the `` massive mobilization of civil society demanding fair elections . '' The U.S. State Department urged the Russian government to conduct an independent , credible investigation of all reported electoral violations , but it also noted steps , like webcams installed at polling places , intended to make the voting process more transparent . Some 90,000 webcams at polling places around the country allowed people around the world to watch live as Russians cast their ballots on election day . Putin called for unity Sunday night as he appeared headed for his third term as president , declaring he had won an `` open and honest fight . '' But chess champion-turned-opposition-activist Garry Kasparov accused Putin 's supporters of `` massive fraud , '' saying early Monday they packed the polls with additional voters . Kasparov , who served as a poll watcher in his Moscow neighborhood , said Putin 's supporters `` simply added new voters to the register using so-called supplementary voter rolls . '' `` At one of the polling stations , the number of extra voters even exceeded the number of registered voters , '' he said . A former KGB officer , Putin served two straight terms in the Kremlin before term limits forced him to step down in 2008 . But he served as prime minister under his successor , Dmitry Medvedev , and continued to dominate Russian politics . Presidents Hugo Chavez of Venezuela and Bashar al-Assad of Syria on Monday congratulated Putin on his win . CNN 's Clare Sebastian contributed to this report .
NEW : Police arrest 250 people in Moscow , including three opposition activists . Thousands take part in rallies both for and against Putin . Vladimir Putin received more than 63 % of the vote . Monitors say they observed ballot stuffing and other irregularities .