Alexei Navalny, a key figure in the rallies after Russia's disputed parliamentary elections, has left jail in Moscow promising new protests.
After serving 15 days for obstructing police, the anti-corruption blogger was freed in the early hours.
A few hours later, the first parliamentary session since the election began in the State Duma.
Police say 25 people were arrested as they staged a rally outside the building, Russian media report.
Speaking to reporters as he left prison, Mr Navalny said "extraordinary efforts" would be made to continue the protest movement.
The protesters' demand for a re-run of the 4 December elections remained in force, he said.
Prime Minister Vladimir Putin's United Russia party won a narrow majority. But activists identified instances of ballot-stuffing and used social media to report them.
Days after Mr Navalny was detained on 6 December, Moscow saw its biggest anti-government protest in decades.
A new mass protest against ballot-rigging is planned on Saturday in Moscow.
"I'm not afraid and these 15 days convinced me there is nothing to fear," Mr Navalny said. "Let them be afraid instead."
Mr Putin is widely seen as favourite to win Russia's presidential election in March.
"The party of swindlers and thieves is putting forward its chief swindler and its chief thief for the presidency," Mr Navalny added. "We must vote against him, struggle against him."