The opposition submitted a petition to President Vladimir Putin on Saturday demanding an investigation into the death of an activist who hanged himself in the Netherlands last week.
Alexander Dolmatov, who had been under investigation for alleged involvement in violence at a protest against Putin last May, hanged himself after being refused asylum by Dutch authorities. He fled Russia in June after police raided his parents' apartment.
On Saturday, about a hundred people joined opposition leaders Sergei Udaltsov and Boris Nemtsov, lining up in the freezing cold to hand in copies of the petition to the presidential administration.
The petition demands the release of all those arrested in the May 6 protest, held on the eve of Putin's inauguration for his third presidential term, and an investigation into the actions of the police. About 400 people were detained and dozens were hurt when riot police and protesters clashed.
According to Russian law, the petition must receive a response within a month. It was handed in without incident, as police stood watching nearby.
Udaltsov and Nemtsov, a former deputy prime minister, said many in the opposition believed that Dolmatov had been forced to leave Russia. Dolmatov's lawyer has said he may have been driven to suicide by threats.
"His nightmare in Holland needs a thorough investigation," Nemtsov said in an interview.
Udaltsov added that Dolmatov's case needed to be investigated on two levels. "We do not remove responsibility from the Dutch authorities," he said. "But it is clear that here in Russia, events had happened that prompted the tragedy."
Police detained eight members of the opposition in a separate unsanctioned Moscow event on Saturday where a group of 30 activists jogged in support of political prisoners, Russian agencies reported.