Moscow (CNN) -- Vladimir Putin, who has dominated Russian politics for more than a decade, was sworn in Monday as the country's president two months after winning back the job in an election clouded by allegations of widespread fraud.
"We're entering a new stage in our national development," Putin said in his inauguration speech. "We'll have to address tasks that are of a totally different level and scale. The next few years will define the future of Russia for decades ahead."
About 2,000 people attended the inauguration under tight security, while nearby, police arrested about 120 people protesting against the return of Putin to the presidency, according to the state-run Ria Novosti news service.
Putin held the presidency for eight years before term limits forced him to step down in 2008. He then became prime minister under his hand-picked successor, Dmitry Medvedev.
One of Putin's first tasks was to nominate Medvedev to become prime minister, according to Ria Novosti.
Putin, a former KGB officer who has worked to stabilize Russia's economy and curtail dissent, won the March presidential election with nearly 65% of the vote.
International observers said there was legitimate uncertainty about the vote, and chess champion-turned-opposition activist Garry Kasparov accused Putin's supporters of "massive fraud."
Under Russia's newly amended constitution, Putin will serve a six-year term and, if re-elected, could remain in power until 2024 -- the longest of any Russian leader since Joseph Stalin, according to Ria Novosti.
On Sunday, hundreds of people, including opposition leaders and a popular blogger, were arrested in anti-Putin protests.
Many chanted slogans such as "Russia without Putin" and "Putin is a thief."
The protest turned violent when a large number of demonstrators veered off the agreed route of the march, and some protesters fought with police.
Clashes broke out, with some protesters throwing objects -- including sticks and at least one flare -- at police.
Authorities responded by "clubbing demonstrators" who briefly broke through police lines, the 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.
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 said.
Twenty officers were injured and three hospitalized in the clashes, according to RIA Novosti.