The United States has "strongly condemned" the detention of hundreds of protesters throughout Russia Sunday, including the country's opposition leader, Alexei Navalny.
Tens of thousands of Russians demonstrated in cities across the country in support of a call by Navalny for accountability among Russia's elite.
OVD-Info, an organization that monitors Russian political repression, said on its website that more than 1,000 people were arrested in the Moscow demonstrations alone.
That number has not been independently confirmed and state news agency TASS cited Moscow police as saying they made about 500 arrests, including Navalny.
He was detained while walking from a subway station to join the rally at Moscow's iconic Pushkin Square.
Reports from the scene say police put him in a truck that was surrounded by hundreds of protesters. 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.
"Detaining peaceful protesters, human rights observers, and journalists is an affront to core democratic values," acting U.S. State Department spokesman Mark Toner said in a statement.
He said the U.S. is "troubled" by the arrest of Navalny, who has announced plans to run for president in the 2018 election.
The protests appear to be the largest coordinated outpouring of dissatisfaction since the massive 2011-2012 demonstrations following a fraud-tainted parliamentary election.
"This is an important event! We came here to express our position as citizens," said one protester who just gave her first name as Alina. "We came to remain citizens of our country."
"By my presence here, I stand against the corruption of the incumbent power," said another protester who only gave his first name as Maxim. "The authorities do not feel like talking to their people, they communicate only through force-applying methods."
Navalny, a Kremlin critic, called the demonstrations after his Foundation for Fighting Corruption released a detailed report earlier this month accusing Prime Minister Dmitry Medvedev of amassing a collection of mansions, yachts and vineyards through a shadowy network of non-profit organizations.
The report has been viewed over 11 million times on YouTube.
There was scant coverage of the demonstrations on Russia's official media. A short report on TASS said a police officer was injured during an "unauthorized" rally in Moscow.
Navalny said on his official website that 99 Russian cities planned to protest, but that in 72 of them local authorities did not give permission.
Navalny has been rallying supporters in major Russian cities in recent weeks.
