Story highlights Thousands protest against President-elect Vladimir Putin and his political allies

Some demonstrators veer off the agreed route and throw objects at authorities

State-media reports police "clubbed" some protesters; police deny using tear gas

More than 250 are arrested, including key opposition figures, police say

An anti-government protest in Moscow turned violent Sunday, with some demonstrators clashing with police after they veered off their agreed-upon route.

More than 250 people were arrested, including leading opposition figure Boris Nemtsov, leftist radical group leader Sergei Udaltsov and popular blogger Alexei Navalny, Moscow police told CNN.

They were among the thousands that descended on the Russian capital for the latest demonstration denouncing Vladimir Putin, the nation's current prime minister who was recently elected to a six-year term as president. He is set to be inaugurated Monday.

As they marched, many chanted slogans such as "Russia without Putin" and "Putin is a thief."

A large number of demonstrators did not follow the route to Bolotnaya Square that their leaders had agreed to with Moscow authorities, instead stopping in front of a line of police and refusing to move.

Clashes broke out, with some protesters throwing various objects -- including sticks and at least one flare -- at police.

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Authorities responded by "clubbing demonstrators" who briefly broke through police lines, the state-run RIA-Novosti news agency reported. One of its correspondents saw "a male protester out cold after clashing with officers," while another reported eight officers arresting one demonstrator "for the apparent crime of wearing a Guy Fawkes' mask," the symbol of the vigilante hacking group Anonymous.

Twenty officers were injured and three hospitalized in the clashes, according to RIA-Novosti, which added that those charged with assaulting police may face up to 10 years in prison if convicted. Itar-Tass, another official news agency, said four officers suffered cuts and wounds after having stones and bottles thrown at them and reporters.

Police denied that they used tear gas as had been reported, according to RIA-Novosti.

After the arrests, authorities slowly moved forward and moved everyone out of the area.

Many protesters expressed disappointment that the event was not peaceful, with some saying they were angry at those who they felt had provoked a confrontation with police.

Others who faced off with authorities voiced frustration that previous peaceful rallies had done little to forward their political aims.

In March, Putin won the country's presidential election with nearly 65% of the vote, claiming victory amid allegations by independent polling monitors of widespread electoral violations. International observers said there was legitimate uncertainty about the vote, with chess champion-turned opposition activist Garry Kasparov accusing Putin's supporters of "massive fraud."

Putin, the former KGB officer who has worked to both stabilize Russia's economy and curtail dissent, held the presidency for eight years before term limits forced him to step down in 2008.

His election followed a series of anti-Kremlin protests in the wake of disputed parliamentary elections that took place in December.