Putin Opponent Detained by Police Amid Massive Protests in Russia

(Photo: REUTERS/Maxim Shemetov)Law enforcement officers scuffles with opposition supporters blocking a van transporting detained anti-corruption campaigner and opposition figure Alexei Navalny during a rally in Moscow, Russia, March 26, 2017.

Hundreds of protesters and opposition leader Alexei Navalny were arrested in a crackdown in the Russian capital Sunday after massive anti-corruption demonstrations were held across the country to demand the resignation of Prime Minister Dmitry Medvedev.

After the protests, which are being seen as the biggest since the anti-Kremlin demonstrations of 2011-2012, police in central Moscow's Tverskaya Street detained Navalny, who is likely to run against President Vladimir Putin next year, according to Reuters.

A police helicopter circled overhead during the arrest. Navalny was put in a truck, which hundreds of protesters crowded around, trying to open its doors, the newswire reported.

"I'm happy that so many people came out (onto the streets) from the east (of the country) to Moscow," Navalny was quoted as saying moments before his detention.

Between 7,000 and 30,000 people demonstrated in Moscow, and up to 10,000 in St. Petersburg, according to ABC News, which also cited Russian human rights group OVD-Info as reporting that more than 700 people were detained in Moscow, 34 in St. Petersburg, and between 80 and 100 in other cities.

Navalny faces a fine or 15 days of detention for organizing a mass rally and violating public order.

"We call on the government of Russia to immediately release all peaceful protesters," State Department spokesman Mark Toner said in a statement.

"Detaining peaceful protesters, human rights observers, and journalists is an affront to core democratic values," Toner continued. "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 the situation, the State Department said.

An American journalist, who was not identified, recorded the protests on camera, and was arrested and charged with "participation in [an] unsanctioned protest," according to U.K. newspaper The Telegraph.

The State Department said it is "ready to provide all appropriate consular assistance."

U.S. Senator Ben Sasse, a member of the Senate Armed Services Committee, also condemned the crackdown.

"Putin's thugocracy is on full display," Sasse said in a statement. "The United States government cannot be silent about Russia's crackdown on peaceful protesters. Free speech is what we're all about and Americans expect our leaders to call out thugs who trample the basic human rights of speech, press, assembly, and protest."