By Gabriela Baczynska

MOSCOW (Reuters) - Russian investigators on Friday charged protest leader Sergei Udaltsov with plotting riots, in a case that has raised alarm among rights activists and Western governments.

The Federal Investigative Committee formally charged Udaltsov in morning questioning, after a pro-Kremlin TV channel broadcast footage from a hidden camera which it said showed him conspiring with a Georgian politician.

Udaltsov faces up to 10 years in prison if convicted in the case, which he and other Kremlin critics say is part of a tightening clampdown on dissent since Vladimir Putin started a fresh six-year presidential term in May amid a wave of protests.

Unlike two lesser-known activists charged in the same case, Udaltsov was not detained but has been ordered to remain in Moscow pending further investigation and possible trial.

One of the two, Leonid Razvozzhayev, 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 arrival at the Investigative Committee building for questioning, the shaven-headed Udaltsov raised clenched fists.

"I'm not guilty of anything, I hold my head high," he said.

Udaltsov was among 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.

Putin, in power since 2000, won a new term in March. Opponents say he has used legislation and criminal cases to curb opposition.

(Writing by Gabriela Baczynska; Editing by Steve Gutterman and Andrew Roche)