Russian police detained at least 200 people including high-profile figures for taking part at an opposition conference.
The weekend forum had just begun in a Moscow hotel when police burst in and said they were detaining everyone.
According to police, coronavirus rules were broken and the event was arranged by an “undesirable organisation”.
The incident comes as authorities continue to crack down on opposition activity as elections loom.
Last month Alexei Navalny, President Vladimir Putin’s most prominent critic, was jailed. He was accused of breaking the terms of a suspended prison sentence while recovering in Germany from nerve agent poisoning he blames on the Kremlin.
Opposition figures from across Russia were taking part in the Moscow conference to discuss strategy ahead of local and parliamentary elections in September.
TV coverage showed participants being bundled into police vehicles.
Police said “a significant portion of participants lacked personal protective equipment” in contravention of coronavirus rules.
The “undesirable organisation” behind the conference was Open Russia, police said.
Open Russia was established in the UK by Mikhail Khodorkovsky, a former oligarch turned opponent of President Vladimir Putin.
Khodorkovsky spent 10 years in jail on charges his lawyers dismissed as fabricated before being pardoned by Mr Putin and moving abroad.
Open Russia is accused by the authorities of breaking a Russian law which prohibits foreign groups deemed political and is among several organisations described as undesirable.
But one of the opposition figures at the conference, Andrei Pivovarov, said it had in fact been organised by a group known as the United Democrats.
He said the raid had been staged “to intimidate us”.
“No-one promised us freedom on a silver platter – Russia will still be free,” another opposition politician at the conference, Ilya Yashin, said on Facebook.
Commenting on the arrests, associates of Mr Navalny said it was clear “the authorities are afraid of any competition during elections”
Those detained are expected to be fined or face a short spell in police detention.
The mass arrests come as officials increasingly refer to opposition activists as anti-Russian, and supposedly backed by the West.