Several Russian opposition leaders have been detained while demonstrating against the claimed torture of fellow activist Leonid Razvozzhayev.
Hundreds of people assembled in central Moscow on Saturday afternoon to protest an increasingly relentless crackdown on the opposition in Russia as well as Razvozzhayev's treatment.
Earlier this week investigators said that Razvozzhayev had turned himself in and confessed to plotting riots.
But days later, he retracted his confession and filed a complaint over what he said was his abduction from Ukraine.
Supporters who visited him in prison claim he had been tortured into confessing under duress.
Opposition leaders Alexei Navalny, Sergei Udaltsov and Ilya Yashin also participated in the protest and held one-man pickets spelling "torture and repression" outside the Russian former intelligence and former KGB headquarters.
Three men and several other activists were detained by police though charges are yet to be brought against them.
Meanwhile, former Russian presidential candidate Mikhail Prokhorov announced that he was leaving his business interests to dedicate more time to politics.
He will be returning to the political arena after remaining silent through a five-month Kremlin crackdown on the opposition.
Addressing reporters after the conference Prokhorov announced his intention to lead "a third power" in the country, competing both with the Kremlin and the opposition.
Business tycoon Prokhorov is believed to be worth about $13bn and finished third in Russia's presidential election in March amid speculation that his candidacy was orchestrated by the Kremlin.
At the time he denied the allegations and was nowhere to be seen as the government launched a crackdown on the opposition this spring, arresting activists and introducing new harsh legislation.
Prokhorov appeared on Saturday at the first conference of the party he set up several months ago, announcing that he would put his money in a trust fund and let his partners at the investment vehicle Onexim run the shop.