Spain's national police said Browder had been detained briefly and taken to a police station in downtown Madrid until agents confirmed that an arrest warrant from Russian Federation that appeared in Interpol's system was outdated.
"Bill Browder remained in police custody this morning for the minimum necessary time, until Interpol Spain verified that the arrest warrant issued by the Russian authorities for tax evasion was invalid", according to a Spanish-language tweet by police.
Mr. Browder, a US -born investor who has sharply criticized the Putin government, was arrested on what Spanish police and Mr. Browder said was a request from Russian Federation. "Going to the police station right now", Browder said on Twitter, adding that the police would not tell him which station he was being taken to.
However, in an email, Interpol said it has never issued a Red Notice for Browder.
"They won't tell me which station", he wrote in a follow-up.More news: Italy's anti-establishment leaders revive governing coalition
Chicago-born Browder is the founder and CEO of Hermitage Capital Management, an worldwide investment fund.
Browder tweeted the news from the back of a police auto, but said he did not know where he was being taken.
A Spanish national police spokesman said the arrest couldn't be immediately confirmed by authorities. As far as we know, he is a British citizen now.
Russian Federation had convicted Browder in absentia twice and both times sentenced him to nine years in prison (on charges of deliberate bankruptcy and non-payment of taxes).
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.More news: Treatment of immigrant children dehumanizing
This resulted in the 2012 passage of the Magnitsky Act, which allows the USA government to blacklist Russian officials "thought to be responsible" for Magnitsky's death.
British foreign minister Boris Johnson said on Wednesday he had spoken to Browder and was glad he had been released.
As early as July 2013, the global police organization said it erased from its database Russian requests to locate and arrest Browder due to its "predominantly political character".
After Hermitage's offices were raided in 2007 and documents relating to a Hermitage-administered company called Kamaya were seized, Browder had Magnitsky investigate what happened.More news: Trump Broke Out a Throwback Lie About Firing Comey