US Secretary of State Antony Blinken on Saturday called for an end to "persecution of independent voices" after around 200 opposition politicians and municipal deputies in Russia were arrested.
While Russian police routinely break up opposition protests, the mass arrests of municipal deputies at a conference in Moscow were unprecedented.
"We call for an end to the persecution of independent voices," Blinken wrote in a tweet as authorities tightened the screws on Kremlin critics ahead of parliamentary elections.
He noted "the Russian government detained almost 200 municipal leaders and politicians, including political activists Vladimir Kara-Murza and Yuliya Galyamina, on dubious grounds."
Participants from more than 50 of Russia's regions had gathered to discuss parliamentary and local elections in September at a forum organized by a project backed by prominent Kremlin critic Mikhail Khodorkovsky.
A number of the detained activists said Saturday evening they had been released but ordered to appear in court at a later stage.