In a fresh episode in crackdown on the opposition, the best-known left-wing leader Sergei Udaltsov, was arrested on accusations of plotting mass riots.

Mr. Udaltsov, a key organiser of thousands-strong rallies against President Vladimir Putin , was detained following a criminal case against him and several activists for their alleged role in preparing “mass disorders”.

The charges, carrying a maximum sentence of 10 years in prison, were based on a recent documentary aired by the Kremlin-controlled NTV television channel which claimed that opposition leaders conspired with Georgian officials and disgraced Russian businessmen to overthrow the government.

Police wearing black masks searched the apartments of Mr. Udaltsov and his parents in Moscow on Wednesday and took him away for questioning.

Mr. Udaltsov denied the charges and insisted the footage presented in the documentary had been doctored.

Kremlin critics condemned Mr. Udaltsov’s arrest a part of a campaign of political reprisals aimed at stifling any public protest.

“This is an act of political backlash and political repression,” said liberal Yabloko Party leader Grigory Yavlinsky. “It is widening the chasm between society and the state and is aggravating the political crisis.”

Another opposition leader, Alexei Navalny, is also facing up to 10 years in jail in a criminal case of embezzlement that was twice closed in the past before being reopened again this year.

Opposition politician Gennady Gudkov, an outspoken critic of the Kremlin, was recently stripped of his parliamentary seat for engaging in business in violation of the MP status.

Investigators on Wednesday opened an embezzlement case against organisers of a nationwide internet vote to elect the most popular opposition leaders.

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