Russian police officers detain an unidentified man during a protest rally in Bolotnaya Square in Moscow. Up to 20,000 opposition supporters gathered for a protest on Monday, venting anger against the Kremlin and demanding the release of political prisoners. (Ivan Sekretarev/AP)

Thousands of protesters crowded a square in central Moscow on Monday to demand political freedoms and an end to corruption, in an attempt to inject new life into Russia’s flagging opposition movement.

The largely peaceful demonstration marked a year since Russian authorities cracked down on the opposition on the eve of Vladimir Putin’s presidential inauguration. Since then, many opposition leaders have been targeted with prosecutions, and some ordinary protesters arrested then are still in jail. But many of the roughly 20,000 people who turned out on a brisk spring evening vowed that they would not stop agitating until Putin yielded to their demands.

“Our main task is to throw out those from our country who want to steal from us,” said Alexei Navalny, a prominent opposition leader and blogger who has declared his desire to run for president. Navalny is on trial on embezzlement charges that he says were filed at Putin’s behest. “I will never surrender and I will never leave,” said Navalny, who denies the charges.

Despite the defiant tone, many at the protest seemed far less optimistic than they did during the first protests in December 2011, when a previously politically apathetic generation turned out into the streets to object to parliamentary election results widely believed to be fraudulent. The demands swiftly expanded. Then, a year ago, authorities scared many people back into their homes by pursuing harsh sentences against ordinary protesters.

One man on Monday carried a sign that read, simply, “Everything is very bad.”

Another, Alexander Romanov, a 65-year-old retiree, was wearing a yellow star with “foreign agent” printed on it, in solidarity with members of nongovernmental organizations who are being made to register as foreign agents if they receive money from abroad. Here the term holds dark connotations of Cold War treachery.

“It’s a stupid law,” Romanov said. He said the authorities’ behavior reminded him of his daughter when she was little. “She held our cat tightly and wouldn’t let it go because she knew as soon as she did the cat would bite her. Some of our leaders are like that. They are holding tightly to power because they know if they give it up, they’ll end up in prison.”

Police estimated crowds at 8,000, Interfax reported, but some media reports put the number as high as 30,000. Opposition leaders said that national holidays may have depressed turnout.

Six people were arrested, Moscow police said. Earlier Monday, an opposition volunteer was killed in an accident during the setup of the stage.

“We just don’t like someone else making the decisions for us. Our patience is worn out completely,” said Maria Smolentsev, 36, a stay-at-home mother who said that she was attending a protest for the first time. She said that the last straw was a ban in January on U.S. adoptions of Russian children, the response to U.S. passage of the Magnitsky list law. The measure bans entrance into the United States and freezes U.S. assets of Russian officials tied to the death in police custody of Sergei Magnitsky, a lawyer who accused officials of embezzlement.

One protester said that despite setbacks for the opposition movement, pressure was not going away.

“Russians say the first pancake comes out all crumbled,” said Alexei Romanchikov, 25, an Internet specialist. “Maybe it’s the beginning.”

Will Englund contributed to this story.