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 organised 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.