Russian opposition leader Alexei Navalny was detained on Monday over allegations of staging illegal protests, his associates aid, adding that the move was meant to sideline him at a time of growing discontent over unpopular government reforms.
Navalny's arrest came moments after he walked free from jail at the end of an previous incarceration, his supporters said.
The Russian Interior Ministry did not immediately reply to a request for comment.
The Kremlin's plan to raise the retirement age has seen thousands of Russians take to the streets in recent weeks and shaved around 15 percentage points off President Vladimir Putin's popularity rating.
Navalny's new detention came after he finished a 30-day stint in jail for planning an unauthorised demonstration in the Russian capital in January that called for a boycott of what he said would be a rigged presidential election.
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He said at the time that his jail sentence was designed by the authorities to prevent him from leading nationwide protests against the pension reform on Sept. 9, where more than 800 people were detained.
Leonid Volkov, a Navalny ally, wrote on Twitter on Monday morning that Navalny had been taken to a police station as he walked out of jail and was again being accused of violating protest laws.
Navalny's case will be heard in court later on Monday and he could face a fine and up to 20 days in jail, his spokeswoman Kira Yarmysh wrote on Twitter.
Lyubov Sobol, a lawyer at Navalny's anti-corruption foundation, said he was being targeted by the authorities because of the pension reform protests.
"They are isolating a politician and leader of the opposition," she wrote on Twitter.
The proposed pension reforms are the most unpopular government measure since a 2005 move to scrap Soviet-era benefits, though Putin softened the plan after initial protests.
Navalny, who has been repeatedly jailed for planning unauthorised demonstrations, has pledged he would never give up trying to organise street protests.