Hundreds of protesters have rallied in central Moscow to demand the release of political activists detained since mass protests.
Protesters at Saturday's rally called for an end to Russian President Vladimir Putin's rule, as they held portraits of the jailed activists and placards with slogans like "Free Hostages".
Fifteen have been put in pre-trial detention over the May 6 rally that descended into violence and resulted in 25 arrests. Another three, including radical opposition leader Sergei Udaltsov, are under house arrest.
The crackdown had occurred on the eve of President Vladimir Putin's return to the Kremlin last year.
The opposition and supporters say the authorities are seeking to prosecute innocent people in a bid to intimidate the rest of the dissenters into obedience.
'For all political prisoners'
Gennady Gudkov, a former opposition parliament deputy who was expelled from the State Duma by Putin loyalists, told the rally that the demonstration was for all political prisoners in Russia.
"There are many dozens and even hundreds," he said of the political prisoners.
"It's not mass repression yet, but we are holding this action in order to stop the possibilty of the mass repression that we already overcame: the most terrbile repression was in the 1930s, when tens of millions were victimised.
"We don't want to repeat the Gulags, we don't want to repeat the tragedy. That's why we're here to say our piece."
In a sign of the scale of the punishments they may face, one of the accused, businessman Maxim Luzyanin, was sentenced last year to four and a half years in prison after he pleaded guilty.
The fate of the activists has become a rallying cause for the opposition movement, which is trying to keep up the momentum after unprecedented protests that shook the Kremlin in 2011 have largely died down.
The opposition plans to hold a bigger rally next month to mark the one-year anniversary of the May 6 protest.
The mass demonstrations in December 2011 began over allegations of fraud in a parliamentary election won by Putin's party and turned into the biggest protests against him since he first became president in 2000, at times drawing up to 100,000 people.
Putin dismisses the protesters as a minority who do not have wide support across Russia and the Kremlin has resisted calls for political change.