Russian police on Saturday arrested around 200 opposition politicians and municipal deputies at a Moscow conference as authorities tighten the screws on Kremlin critics ahead of parliamentary elections.

A police raid on an opposition conference dedicated to running for municipal office came after President Vladimir Putin's top critic Alexei Navalny was jailed for two and a half years last month and more than 10,000 protesters detained across the country.

While Russian police routinely break up opposition protests, the mass arrests of municipal deputies at a conference in Moscow were unprecedented.

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.

Forty minutes into the conference police broke up the event and detained the participants.

"The ENTIRE forum of Russian municipal deputies has been detained in Moscow!" Khodorkovsky said on Twitter, calling the detentions "unconstitutional."

A number of well-known opposition figures including Yashin, Vladimir Kara-Murza, Yulia Galyamina, Yevgeny Roizman and Andrei Pivovarov as well as journalists have been detained.

"A very symbolic end to a short forum: deputies in police vans, and masked police are twisting people's arms," Yashin said on Facebook.

Moscow police said in a statement that around 200 people had been detained.

Many of the conference participants did not wear masks, while some were members of an organisation whose work had been declared "undesirable," police said.

Pivovarov, speaking to AFP from a police station, said the forum participants had been detained because authorities believe that the conference was organised by Open Russia, a movement founded by Khodorkovsky and designated as an "undesirable organisation."

The municipal forum -- the first of its kind -- was organised by United Democrats, another project backed by Khodorkovsky, Pivovarov said. He noted that authorities appeared to look for a pretext to interrupt an opposition event.

"Their goal is to intimidate us," he said.

Khodorkovsky's Open Russia was banned in Russia in 2017 in line with a controversial law that targets foreign groups accused of political meddling.

Entities put on the "undesirable" list are banned from issuing any publications in Russia, while people cooperating with them could be hit with fines, jail time and Russian entry bans.

Khodorkovsky, who owned the oil giant Yukos before he was convicted in two controversial cases and spent a decade behind bars, now lives abroad.

'Politics is crime'

The team of jailed Kremlin foe Navalny accused authorities of seeking to further intimidate critics ahead of the September elections.

"It is clear why the forum has been broken up -- authorities are afraid of any competition during elections," the team said on the Telegram messaging app, adding that the ruling party United Russia was getting increasingly unpopular.

"Winning even rigged elections is becoming ever more difficult."

Kremlin critics accuse Putin of steadily ramping up pressure on critics during his two decades in power, sidelining the opposition and tightening control on everything from the main television stations to parliament.

The mass detentions of local deputies could mark a new chapter in the authorities' crackdown on dissent, political observers said.

"Politics in Russia is a crime, now it's official," tweeted Kirill Martynov, politics editor at Russia's top independent newspaper Novaya Gazeta.

Political analyst Abbas Gallyamov said the forum could have gone unnoticed if it were not for Russia's law enforcement.

"No one is really interested in the subject of local governance," Gallyamov wrote. "Now it's a full-fledged political event."