Alexei Navalny, the leader of protests against President Vladimir Putin that resulted in the arrests of more than 1,500 demonstrators across Russian Federation including himself, said on Sunday that the actions were worthwhile even though he faces another possible stretch in jail.
Mr Putin, 65, was re-elected in a landslide victory in March - extending his grip over the world's largest country until 2024.
For his third and now fourth spells as president, the term was extended to six years from four.
Officials and analysts say bold policies with the best chance of reviving growth have been mired in disagreements among Putin's policymakers.
In a speech after the swearing-in ceremony, Putin said that in the next six years Russian Federation would prove a strong, muscular player on the world stage, backed by a powerful military, while pushing hard to improve life for its citizens at home.
In a new development that shocked many, police in Moscow were helped by pro-Putin activists dressed as Cossacks, a paramilitary class who served as tsarist cavalrymen in imperial Russian Federation.
"Russia should be modern and dynamic, it should be ready to accept the call of the times", he said.
The protests and Navalny's arrest and subsequent release all come just ahead of Putin's inauguration Monday. Allegations of ballot-rigging had dogged previous elections too.
Putin is preparing to begin his fourth term as Russian president.
To foes, however, Putin has dragged his homeland further from democracy, presided over a seizure of the state by a new elite of former secret police cronies and stoked nationalism in a bid to restore Moscow's lost empire.
Mr Navalny was arrested briefly as he tried to join Saturday's unauthorised protest rally in Moscow under the slogan "He's not our tsar".
In line with the constitution, when Putin ended his second term in 2008, he handed power to protege Dmitry Medvedev and moved to become prime minister - but few doubted who was really in charge. He is backed by state TV and the ruling party.
Russian president-elect Vladimir Putin walks prior to his inauguration ceremony at the Kremlin in Moscow on May 7, 2018.
His decision to annex Crimea from Ukraine in 2014 sparked one of the biggest worldwide crises since the Cold War, incurring Western sanctions which continue to this day.
"We need trust in our society".
When Yeltsin sensationally resigned on New Year's Eve 1999, Putin took over as president of the biggest country on Earth.
But Independent political analyst Dmitry Oreshkin said the president's approach to the worldwide community would have to shift over the next term.