* Putin critics say Kremlin orchestrating crackdown on dissent
* Sergei Udaltsov not kept in custody after being charged
By Gabriela Baczynska
MOSCOW, Oct 26 (Reuters) - Russian investigators on Friday charged an outspoken opponent of President Vladimir Putin with plotting riots - allegedly after conspiring with a Georgian politician - in a case rights activists say is designed to suppress dissent.
The federal Investigative Committee formally charged Sergei Udaltsov after questioning him about allegations based on hidden-camera footage broadcast by a pro-Kremlin channel that said it showed him conspiring with a Georgian politician.
Udaltsov, who was released after being charged, but faces up to 10 years in prison if convicted, denied the allegations. "I repeated that I did not plan, prepare or organise mass riots - I am a supporter of peaceful mass protests," he said.
Kremlin critics say the case is emblematic of a clampdown 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 veteran human rights activist Lyudmila Alexeyeva. "The tightening of screws is continuing: the planned annihilation of the opposition."
Udaltsov was ordered to remain in Moscow pending further investigation. Two lesser-known activists charged in the same case have been arrested, including Leonid Razvozzhayev, who said he was abducted in Ukraine last week and tortured. He has formally retracted his confession, his lawyer Mark Feigin said on Thursday.
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.
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.
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 make inroads into the former KGB spy's grip on power.
Putin's critics say he has used new laws and criminal cases to quash dissent since his inauguration.
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.
Another protest leader, Alexei Navalny, could be jailed for 10 years in a separate case on financial-crimes charges he says are politically motivated.
Putin has signed laws increasing fines for violations at street demonstrations 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," said Alexeyeva, 85, a Soviet-era dissident.
"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."