Alexei Navalny left a detention center in western Moscow on Friday after serving 15 days for handing out leaflets in the city's metro for a demonstration planned for March 1.
He was found guilty of violating the law and organizing an illegal gathering.
Navalny's release came a week after fellow anti-government activist Boris Nemtsov was shot and killed in what his supporters have said was an attempt to intimidate opposition to the government.
The rally became an impromptu mourning march for Nemtsov, who died on February 27 on a bridge near the Kremlin.
Navalny had asked to be released early in order to attend Nemtsov's funeral on Tuesday, but his request was denied.
The Kremlin critic was the driving force behind massive protests in 2011-12 against Russian President Vladimir Putin.
The government has since clamped down on anti-Kremlin sentiment, tightening the rules around public gatherings.
He was given a three and a half year suspended sentence in December for fraud and ordered to stay under house arrest. He has already violated the order multiple times.
Navalny's brother Oleg was jailed on similar charges, which many believe are politically-motivated. On Wednesday, Oleg was moved from a Moscow jail to a prison elsewhere in Russia.
Putin has denied any connection to the case against the Navalnys, or Nemtsov's murder.
an/kms (Reuters, AP)