An outspoken opponent of Russian President Vladimir Putin has been charged with plotting riots in a case rights activist say is meant to crush dissent.
Sergei Udaltsov, 35, was charged on Friday in Moscow by investigators who say he conspired with a politician in neighbouring Georgia to stage the protests.
The federal Investigative Committee formally charged Udaltsov after questioning him about allegations based on hidden-camera footage broadcast by a pro-Kremlin channel that said it showed him conspiring with the politician.
Two men have already been charged in the probe, including Leonid Razvozzhayev, an aide to an opposition parliamentary legislator.
Razvozzhayev has retracted a confession that he says he made under duress after being kidnapped apparently by Russian security agents in Ukraine.
Udaltsov, who denied the allegations, was released after being charged but faces up to 10 years in prison if convicted.
He was one of the leaders of a series of protests prompted by allegations of fraud in a December 2011 parliamentary election won by Putin's ruling United Russia party.
"I repeated that I did not plan, prepare or organise mass riots - I am a supporter of peaceful mass protests," he said.
Government critics say the case is part of a wider crackdown on dissent since Putin started a new six-year term in May amid a series of protests against his rule.
"I think this is part of a plan readied long ago to quell the protest wave," said Lyudmila Alexeyeva, a veteran human rights activist.
"The tightening of screws is continuing: the planned annihilation of the opposition."
Udaltsov was ordered to remain in Moscow pending further investigation.
Several people gathered outside the Investigative Committee building in Moscow to support Udaltsov. One held a banner reading: "I demand an end to repression and torture."
On his arrival at the building for questioning, Udaltsov raised clenched fists in a symbol of strength and victory.
The protests drew tens of thousands of people into the streets of Moscow, underscoring dismay among some Russians with Putin's nearly 13-year rule, but the opposition has since then failed to loosen the former KGB spy's grip on power.
Several people face potential prison sentences over violence that erupted between police and protesters at a rally the day before Putin's inauguration on May 7.
In July, another protest leader, Alexei Navalny, was charged with theft after the investigative committee alleged he organised a scheme to steal assets from a state timber company.
The assets are estimated to be worth about $500,000. Navalny dismissed the allegations as "weird" and baseless.
Putin has signed laws increasing fines for violations at street protests and tightening controls on foreign-funded non-governmental organisations that Russia accuses of meddling in its domestic politics.
"We sense some hysteria at the top," Alexeyeva, 85, a Soviet-era dissident, said.
"They don't know a way to silence dissent other than to threaten people by coming up with more and more new legal bans and limitations. Soon they will ban us from breathing."