Russian investigators opened a criminal probe against leftist leader Sergei Udaltsov and several others today for allegedly plotting mass riots.
Russia's top investigative agency said that it will also check claims made in a recent TV documentary that opposition leaders worked with Georgian officials to arrange terrorist attacks across Russia.
Armed men wearing ski masks searched Mr Udaltsov's flat for more than five hours today.
His parents' home was also searched.
A documentary last week broadcast footage purporting to show the Left Front leader meeting officials from Georgia to discuss raising £120 million for protests against President Vladimir Putin and riots in Moscow.
Despite political differences with Mr Udaltsov, the Russian Communist Party dismissed the allegations as nonsense.
Party leader Gennady Zyuganov said that Mr Udaltsov is being persecuted for his views.
He said: "The main goal is to nip the protests in the bud. There's no-one in the Kremlin who can deny that."