Bill Browder, the UK-based financier and anti-Putin campaigner, was taken to a Madrid police station on Wednesday morning after being detained on a Russian warrant.

Mr Browder, who describes himself as "Putin's number one enemy", tweeted that he had been arrested by Spanish police on a Russian Interpol warrant, posting pictures from the back of a police car.

He was released two hours later.

The American-born businessman, who has held British citizenship for the past two decades, was last year sentenced by a Russian court to nine years in prison on fraud and tax evasion charges.

The detention of Mr Browder - widely considered the creator of the Magnitsky Act, an international sanctions regime against Russian officials named for his lawyer who died in Russian custody - drew immediate international alarm.

Guy Verhofstadt, the European Parliament's Brexit negotiator, said it was "worrying that autocratic Russia can get democratic Spain to go after someone fighting to expose Putin's crimes & those responsible for Magnitsky's murder".