Russian opposition leader Alexei Navalny said on social media on Sunday he had been released from detention after spending around three weeks in jail.
Navalny was detained by police in late September and sentenced to 20 days in jail on charges of repeatedly violating laws by organizing public meeting and rallies.
After release, Navalny also said on social media he planned to speak at a political meeting in the provincial town of Astrakhan in southern Russia later on Sunday.
In Moscow, a banner hung on a bridge near the Kremlin on Sunday, calling for people to vote for Navalny and get rid of President Vladimir Putin.
Navalny hopes to run for president in 2018 even though Russia's central election commission declared him ineligible because of a suspended prison sentence, which he says was politically motivated.
Earlier this month Europe's top human rights court ruled Navalny's conviction for fraud in 2014 had been "arbitrary and manifestly unreasonable" and ordered Russia to pay him compensation.
A suspended jail sentence in a separate case -- which Navalny says was politically motivated -- may still bar him from running for president.
Putin has for months declined to say whether he would run for what would be his fourth stint in the Kremlin. If he runs, he is widely expected to win the election.
Another candidate is Russian TV personality Ksenia Sobchak who said earlier this week said she planned to run in the election due in March, 2018, offering liberal voters unhappy with Putin's rule someone to back, though she has little prospect of winning.