Hundreds of people including top Kremlin critic Alexei Navalny were arrested on Sunday as thousands of Russians defied bans to stage protests across the country against corruption.
Navalny had called for the marches after publishing a detailed report this month accusing Prime Minister Dmitry Medvedev of controlling a property empire through a shadowy network of nonprofit organisations.
The report has been viewed over 11 million times on YouTube, but so far Medvedev has made no comment on the claims.
Sunday's march in Moscow was one of the biggest unauthorised demonstrations in recent years, with police putting turnout at 7,000-8,000 people.
Police detained Navalny, who has announced plans to run for president in the 2018 election, as he was walking to the protest, putting him in a police minibus.
The crowd briefly tried to block it from driving off, shouting "Shame!" and "Let him out!"
"Guys, I am all right, go on along Tverskaya," Navalny tweeted from the van, referring to Moscow's main central street.
Police said about 500 people had been arrested in Moscow, while OVD-Info, a website that monitors the detention of activists, said at least 933 had been detained, as well as dozens in other cities.
The Interfax news agency said 130 people were arrested in Saint Petersburg, where about 4,000 people gathered in the city centre.
"We're tired of the lies, we have to do something," Sergei Timofeyev, a 23-year-old protester in Saint Petersburg, told AFP.
A spokeswoman for Navalny's Anti-corruption Foundation (FBK) said on Twitter than he would be held overnight before being brought before a judge on Monday.
He will face charges of calling a rally that disrupted public order, and could be held for up to 15 days.
From his cell, Navalny posted on Twitter that he was "proud" of the protesters and said the mass detentions were "understandable".
"The thieves defend themselves this way. But you cannot arrest everyone who is against corruption. There are millions of us," he wrote.
The United States condemned the arrests as an affront to democracy.
"Detaining peaceful protesters, human rights observers, and journalists is an affront to core democratic values," acting State Department spokesman Mark Toner said in a statement.
- 'They steal and lie' -
Thousands of people filled central Pushkin Square, some shouting "Russia without (President Vladimir) Putin". Some climbed on to lamp posts and the monument to poet Alexander Pushkin, shouting "impeachment!"
Dozens of police vans and rows of riot officers were lined up as a police helicopter hovered overhead.
"We have all seen the movie, it gives specific examples of corruption, and there has been no reaction," Nikolai Moisey, a 26-year-old factory worker, said of the claims against Medvedev.
"They steal and they lie but still people will be patient to the end. The protest is a first push for people to start acting."
Police officers moved to detain protesters and clear the square, with some using truncheons and pepper spray to disperse the crowd, AFP correspondents said.
Police also searched FBK offices over alleged incitement to hatred, and "Everyone was detained and brought to the police," the organisation's spokeswoman Kira Iarmych said.
- '99 cities' -
Despite the dramatic scenes in Moscow, state TV did not cover the protests, instead showing soap operas and nature films.
"The whole country is tired of corruption on such a scale," 50-year-old Natalia Demidova said. "Medvedev should be fired once such exposes come to light."
Elsewhere, about 2,000 gathered in the Siberian city of Novosibirsk, carrying signs such as "No to corruption," according to local news website Sib.fm.
Some held up images of yellow rubber ducks, following reports that Medvedev has a special house for a duck on one of his properties.
Local media estimated about 1,500 people turned out in each of the Siberian cities of Krasnoyarsk and Omsk.
The Russian constitution allows public gatherings, but recent laws have criminalised protests unauthorised by city authorities, who frequently refuse to grant permission for rallies by Kremlin critics.
Navalny said on his website that 99 Russian cities planned to protest, but that in 72 of them the local authorities did not give permission, citing reasons such as street cleaning, a bell-ringing concert and rival events by various pro-Kremlin groups.
There were also protests in Vladivostok and Yekaterinburg, according to local reports.
Navalny, a 40-year-old lawyer by training announced plans to run for the presidency after he won a surprise 27 percent of the vote in the Moscow mayoral election in 2013.
But he has been the subject of several legal prosecutions in recent years, and in February he was found guilty of embezzlement and given a five-year suspended sentence which could make him ineligible to run in next year's vote.