Russian investigators on Tuesday raided the homes of opposition activists said to have attended seminars abroad on how to spark a revolution aimed at toppling President Vladimir Putin.
Investigators raided the homes of opposition activists Taisiya Alexandrova, Yuri Nabutovsky and Anna Kornilova, confiscating documents as well as computers, Investigative Committee (SK) spokesman Vladimir Markin said in a statement.
The Investigative Committee, Russia's powerful equivalent of the American FBI, has already launched criminal probes against a string of top activists over the allegations first broadcast in a smear documentary by pro-Kremlin television.
The documentary on NTV called "Anatomy of a Protest-2" said that activists were plotting an anti-Putin uprising with funding from Georgian lawmaker Givi Targamadze.
Markin said investigators had discovered that a group of Russians visited EU member Lithuania in spring this year to take part in a seminar on how to trigger a revolution in Russia.
The seminar was devoted to "the example of usurping power through colour revolutions" like those that ousted post-Soviet regimes in Ukraine and Georgia in recent years, he said.
A statement by investigators on the SK's website said Alexandrova, Nabutovsky and Kornilova had all participated in such seminars in the past.
Some of the electronic materials seized from the three in the searches on their homes included "literature on seizing power", it added.
The three had now been taken to the headquarters of the Investigative Committee for further questioning, it said. They have been formally named as witnesses in the criminal probe.
Russia has always regarded with the greatest suspicion the "colour revolutions" such as the 2004 Orange Revolution in Ukraine or the 2003 Rose Revolution in Georgia.
Both brought to power pro-Western leaders who sought to move their nations away from Moscow's influence and also sparked Kremlin worries of similar events in Russia itself one day.
Nabutovsky -- an activist with the opposition Solidarity movement -- told Moscow Echo radio that the investigators had shown up at his home in Moscow at 7:00 am (0300 GMT).
He spoke to the radio station as the raid was being carried out, with a voice in the background heard shouting that he could call a lawyer and then telling him to hang up.
One of the anti-Putin movement's most prominent figures, the Left Front leader Sergei Udaltsov, has already been charged with seeking to organise mass rioting but has not been arrested.
However, the authorities are holding in detention opposition activist Leonid Razvozzhayev who claims that security forces abducted him in Ukraine and took him back to Russia.
Markin said that the financing from Targamadze for Udaltsov and Razvozzhayev's alleged attempts to stir up mass riots had now been proven.
Udaltsov was himself summoned Tuesday to the Investigative Committee and told reporters afterwards that he had been made aware of video analysis of the NTV documentary pictures.
"It's clear that they will again ask me in for questioning so no-one intends to leave me in peace," he said, quoted by Russian news agencies.
Udaltsov added he is still obliged to remain in Moscow as the investigation continues and cannot divulge any details of the probe.
Meanwhile, other raids took place in the city of Nizhny Novgorod, 500 kilometres (310 miles) east of Moscow. Activists Yekaterina Zaitseva and Alexander Averin said that they were both targeted.
The new raids coincide with the anniversary of the mass opposition protests against Putin's rule that erupted in December last year which were the first big challenge to his hold on power.
The Russian opposition is planning a new anti-Putin mass rally on Saturday which will be a crucial test of whether there is still life in the protest movement one year on.
Putin returned to the Kremlin in May for a third mandate as Russian president following his four-year spell as prime minister and activists have complained they have since been subjected to an unprecedented crackdown.