Riot police in Russia have detained four opposition leaders, preventing them from taking part in a banned rally against President Vladimir Putin.
The demonstrations in the capital Moscow marked the first anniversary of anti-Putin protests after parliamentary elections.
Ilya Yashin and Ksenia Sobchak were held on their way to the march, while Sergei Udaltsov and Alexei Navalny were among around 40 protesters who were reportedly detained in the central Lubyanka Square.
Up to 3,000 people gathered there, braving temperatures of -14C (7F).
"I am proud of each and every one of those who came," Mr Navalny said. "The main thing is that people are here, that they are expressing their view and showing they exist."
There was a heavy police presence in the square, which holds a memorial to victims of political repression during Soviet times.
After laying flowers at the scene, Boris Nemtsov, a veteran Russian politician said: "The people who have come here are free, honest and decent people.
"I'm very proud of our people, of Muscovites, of Russians. They (the government) wanted to scare us, there's a helicopter flying over us and they've surrounded us with policemen. They think that we're slaves, but we're not. We're free people, and thank God for that."
One protester, who brought her prayer book with her to the protest, said: "I'm praying for Russia. God made us free. No one can take that away from us or punish, detain or torture us for our political views."
The protests began last December amid accusations of vote-rigging by Mr Putin's United Russia party.
Opposition leaders accuse the president of clamping down on dissent and freedom of expression, something he denies.
Laws passed since Mr Putin began his third term in May increase the punishment for protesters who step out of line and tighten control over the internet.
Several opposition leaders face criminal charges, although they claim these are politically motivated and intended to intimidate them into giving up their opposition activities.