Investigative Committee spokesman Vladimir Markin said the agency had gathered new evidence confirming earlier allegations that leftist activists Sergey Udaltsov, Leonid Razvozzhayev, and Konstantin Lebedev had met with foreign officials to plot mass riots, inspired by the "color revolutions" that ousted authoritarian leaders in several ex-Soviet nations and rattled the Kremlin.
He said their supporters also attended special training abroad to prepare for "organizing and conducting mass riots with the purpose of overthrowing the government."
President Vladimir Putin has frequently accused the unprecedented protest movement that sprung up against his rule last winter of being funded and directed from abroad.
Charges were filed against the activists in October after pro-Kremlin TV showed a documentary-style film alleging they met officials from Georgia in Minsk to plot violent disturbances across Russia. They deny all charges.
Udaltsov told the Interfax news agency Friday that the new allegations were an attempt to discredit the opposition before a major protest planned for December. "This is the latest pile of garbage they're trying to smear on us," he added.
Razvozzhayev's case became a cause celebre for the opposition after he claimed he was kidnapped last month in Ukraine where he was seeking asylum, tortured for two days and forced to sign a false confession, which he later retracted. Authorities say he turned himself in and deny allegations of torture.
Razvozzhayev has remained in custody since his arrest in late October. Last week, investigators refiled 15-year-old charges claiming that he stole 500 fur hats in Siberia and combined them with the riot-plotting case, as well as claiming Razvozzhayev entered Ukraine illegally on his cousin's passport. His lawyer said the charges were an attempt to scare Razvozzhayev with the prospect of being transferred from Moscow to a jail in Siberia, and added that he had received an anonymous letter containing threats.
Recently, the pro-government newspaper Izvestiya also claimed that Razvozzhayev cheated on his wife with a fellow activist and alleged the existence of a video proving it. Razvozzhayev's lawyer, Anna Stavitskaya, has dismissed the claims as part of a smear campaign aimed at damaging his reputation.