Alexei Navalny, a key figure in Russia's protest movement against President Vladimir Putin, left a Moscow detention facility early on Thursday after serving 15 days for disobeying police orders.
Speaking after he left the facility, Navalny said he would join a makeshift protest camp in downtown Moscow later on Thursday.
Anti-Putin activists settled on the central Arbat street last week after Moscow police broke up two attempts to establish a round-the-clock camp elsewhere in the Russian capital.
Navalny, who made his name as an anti-corruption blogger before becoming the figurehead of this winter's uprecedented mass anti-government rallies, was detained during the protests that followed Putin's May 7 inauguration and was sentenced to 15 days.
Hours earlier, fellow opposition leader Sergei Udaltsov was also released after 15 days behind bars.
Left Front movement leader Udaltsov was sent to jail after being detained for a third time in as many days on May 8.
He had earlier been detained at a mass protest in Moscow on the eve of Putin's return to the Kremlin. That rally saw serious clashes between police and demonstrators and some 400 arrests.
"All attempts by the illegitimate authorities to intimidate us are futile, because we know that we are in the right," Udaltsov told journalists after leaving jail. "We will win."
His lawyer Violetta Volkova said on Wednesday Udaltsov might face immediate re-arrest upon his release on charges of inciting mass disorder.
"A source has informed me that they plan to detain Udaltsov for 48 hours right after he gets out," she said.
Socialite television presenter Ksenia Sobchak said earlier this month that Kremlin sources had told her the authorities plan to jail both Udaltsov and Navalny for two years over the downtown Moscow riots ahead of Putin's inauguration.
