MOSCOW (AFP) - Police in Moscow on Saturday said they had detained around 40 demonstrators protesting apparent pressure from authorities to silence Russia's top opposition broadcaster.

Some 150 people attended a flash mob protest close to the Kremlin in support of Dozhd (TV Rain), an independent Internet and cable channel known for its critical coverage of President Vladimir Putin, an AFP photographer at the scene said.

Police stepped in shortly after participants unfurled umbrellas in a show of support for the beleaguered channel, saying that the protest had not been authorised.

Dozhd has warned that it is facing closure after a string of providers recently dropped the channel from their television packages in what the station, which depends on advertising revenue, called a campaign of intimidation.

The pressure on Dozhd began late last month after it conducted a phone-in poll asking whether Leningrad should have been surrendered in World War II to save hundreds of thousands of lives during the siege by Nazi forces.

Senior pro-Kremlin lawmakers called the poll unpatriotic and asked prosecutors to probe the station over possible extremism.

Dozhd swiftly apologised and pulled the poll, but Putin's spokesman Dmitry Peskov told the station it had "crossed all the limits of what can be tolerated." Editors at the channel said they had now lost about 85 per cent of their audience.

Dozhd management said the poll was a pretext for a crackdown. They linked the pressure to a report on an investigation by top opposition leader Alexei Navalny into luxury homes owned by ruling party bosses.