British businessman Bill Browder, a prominent Kremlin critic, was detained and then released by Spanish police on Wednesday, after a warrant for his arrest he said came from Russia was found to be no longer valid, a police source said.
The head of investment fund Hermitage Capital Management, Browder led a campaign to expose corruption and punish Russian officials he blames for the 2009 death of Sergei Magnitsky, who he employed as a lawyer.
A Russian court sentenced Browder to nine years in prison in absentia in December after finding him guilty of deliberate bankruptcy and tax evasion.
Browder said on his Twitter account that the Interpol General Secretary had advised Spanish police not to honor a Russian Interpol red notice - a request to locate and provisionally arrest someone pending extradition.
"This is the sixth time that Russia has abused Interpol in my case," he tweeted, following his release from a Madrid police station.
A police source could not confirm that the arrest warrant came from Russia or what made it no longer valid. The Kremlin did not immediately respond to a request for comment.
Browder is in Madrid to give evidence to Jose Grinda, a Spanish prosecutor who spearheads investigations into organised crime, about money from the Magnitsky case that has flowed to Spain, the British businessman said on Twitter.
Magnitsky was arrested in 2008 shortly after alleging that Russian officials were involved in large-scale tax fraud. His death nearly a year later while awaiting trial caused an international uproar.
Browder has accused Russian President Vladimir Putin of a personal vendetta over the businessman's efforts to get other countries to impose so-called Magnitsky sanctions against Russian individuals.
British foreign minister Boris Johnson said on Wednesday he had spoken to Browder and was glad he had been released.
"Moscow should concentrate on bringing those responsible for the murder of Magnitsky to justice," Johnson said in a tweet.