The US department of state has "strongly condemned" the detention of hundreds of protesters throughout Russia including the country's opposition leader on Sunday
Thousands took part to anti-corruption protests swept across 80 Russian towns and cities in what are believed to be the biggest demonstrations in the country since 2012, with the largest held in Moscow and St Petersburg.
Alexei Navalny, a campaigner who hopes to stand against Vladimir Putin in next year's elections, was bundled into a police van on Sunday morning as he approached a group of protesters in Moscow's Pushkin Square.
The US department of state said on Sunday it was "troubled" to learn of the arrest of Mr Navalny and called the detention of peaceful protesters "an affront to democratic values."
The United States strongly condemns the detention of hundreds of peaceful protesters throughout Russia on Sunday," spokesman Mark Toner said in a statement on Sunday evening.
Detaining peaceful protesters, human rights observers, and journalists is an affront to core democratic values.
We were troubled to hear of the arrest of opposition figure Alexei Navalny upon arrival at the demonstration, as well as the police raids on the anti-corruption organization he heads.
The United States will monitor this situation, and we call on the government of Russia to immediately release all peaceful protesters.
The Russian people, like people everywhere, deserve a government that supports an open marketplace of ideas, transparent and accountable governance, equal treatment under the law, and the ability to exercise their rights without fear of retribution.
An American journalist captured the incident on camera, only to be arrested himself and subsequently charged with "participation in [an] unsanctioned protest."
The department of state said that it was "aware" of the reports and that it stood "ready to provide all appropriate consular assistance."
The protests were triggered by a film produced by Mr Navalny which claims that prime minister Dmitry Medvedev has seized a collection of yachts, several mansions and a vineyard through questionable means.
Mr Medvedev is also alleged to have used a network of charity websites run by business associates to conceal his links to the deals.
It came after hundreds of demonstrators were also arrested in Belarus over the weekend during a protest in Minsk against a tax on the unemployed.
Around 200 demonstrators were arrested in the Russian capital, according to Russian state media, adding that 25 had been detained in the eastern port city of Vladivostok.
Eyewitnesses told Reuters that four people were also detained at a rally in Yekaterinburg in the industrial Urals region. On Yekaterinburg's Labour Square protesters waved posters reading "We are the authorities here" while nationalists and supporters of the pro-Kremlin United Russia party looked on.
Some eyewitnesses complained of heavy-handed tactics, with one image of the scenes in Moscow showing a young woman being restrained by at least five riot policemen.
Pictures and videos of the protesters posted on social media suggested a large number of then were youngsters, with some looking no older than 14 or 15.
Police estimated the size of the crowds in Moscow at around 7,000, though protesters said the actual numbers were far larger. Sunday's protests marked the largest outpouring of anger with Vladimir Putin's government since the 2011-2012 demonstrations which followed an election dogged by fraud claims.
Anti-corruption protests called by Navalny in cities across Russia today. Riot police drag away protesters in Moscow pic.twitter.com/xV9iMOUCmL— Paula Chertok�� (@PaulaChertok) March 26, 2017
"People are unhappy with the fact that there's been no investigation," protester Ivan Gronstein told the Associated Press.
Denis Korneev, a 17-year-old film student, said: "I've come out (to protest) against corruption and want the authorities to answer the accusations in the Navalny film."
"In many countries the government would have resigned over this." Several demonstrators painted their faces green, a reference to an incident on March 20 when Mr Navalny had green dye thrown in his face by a pro-government activist.
"Navalny has united people who think the same; that people don't agree with the authorities is obvious from what is going on in the country today," Anna Ivanova, 19, told AP. "I am a bit scared."