The United States has strongly condemned Russia's treatment of protesters after the largest anti-government protests in years swept the country.

Amid growing anger at government corruption, thousands of Russians gathered in cities nationwide on Sunday in a rare show of defiance towards their president Vladimir Putin.

The crowds included thousands of young Russians, many of whom have lived their entire lives under Putin's rule.

Riot police responded by detaining hundreds of people including Alexei Navalny, the opposition leader and one of Putin's most prominent critics.

As images and video of the crackdown spread across social media, US politicians called on the Trump administration to respond with a robust message.

"Putin's thugocracy is on full display...Americans expect our leaders to call out thugs who trample the basic human rights of speech, press, assembly and protest," said Republican Senator Ben Sasse.

Riot police detained hundreds of protesters. Credit: AP

The US State Department eventually issued a statement late on Sunday, calling the Kremlin's handling of the protesters an "affront to core democratic values".

"The United States strongly condemns the detention of hundreds of peaceful protesters throughout Russia on Sunday," said Mark Toner, the acting spokesman for the department.

Mr Toner said the United States would continue to monitor the situation and pressure Russia to immediately release all peaceful protesters.

President Trump, who has previously lavished praise on Putin, has not commented on the protests.

The Kremlin's crackdown will further complicate the president's ambition to forge warmer relations between the US and Russia.