Alexander Lebedev was said to be "surprised" by the armed raid, which was part of an ongoing criminal investigation into alleged fraud at Rossiyskiy Kapital, a small bank that Mr Lebedev used to control.

Sources close to Mr Lebedev, 49, were anxious to stress that it was not the oligarch's own bank, the Moscow-based National Reserve Bank, that was under investigation.

He briefly owned Rossiyskiy Kapital and helped it weather a liquidity crisis in 2008. It is alleged that £100 million was fraudulently taken out of that bank before Mr Lebedev came to its rescue and it is that alleged fraud that the police are investigating.

Novaya Gazeta, an opposition newspaper part-owned by Mr Lebedev, said the masked men had first demanded to speak to the oligarch, that his mobile phone had then been mysteriously cut off, and that police officers had later threatened to "shut him down".

Up to 50 officers took part in the raid, leaping over turnstiles at the National Reserve Bank before carrying out a frantic search of filing cabinets for documents connected with the case.

"Apart from the rudeness, it is surprising that nobody asked Lebedev to hand over any documents to investigators before this happened," the newspaper wrote. "Wearing masks for such operations has already been banned and their use this time round leads one to believe that the real reason for this raid was something else."

The newspaper added that it was Mr Lebedev who had reported the alleged fraud to the police.

Mr Lebedev's spokesman told local media that the oligarch's only real enemy was Yuri Luzhkov, the former mayor of Moscow, but did not accuse him outright. "It is incomprehensible why there was a need to put on such a show with masks and guns," the spokesman, Artyom Artyomov, said. "They could have just quietly turned up and checked everything they wanted."

Mr Lebedev, who also owns the London Evening Standard, which he bought for a token £1 in 2009, occupies an ambiguous position in Russian public life.

Although he has often spoken out in broad terms against corruption and abuse of government power, he has been careful to avoid directly criticising Vladimir Putin, the prime minister, or President Dmitry Medvedev.

A former KGB spy who once worked at the Soviet embassy in London, Forbes magazine says he is worth £1.25 billion, putting him among the world's 500 wealthiest people.