Bill Browder, a critic of Russian President Vladimir Putin who heads a UK investment fund, has been briefly detained in Spain on a Russian warrant, which authorities later said had expired.
Browder tweeted on Wednesday that he was being arrested, posting a photo on his official profile saying that he was in the back of a Spanish police car on the way to a police station.
"Urgent: Just was arrested by Spanish police in Madrid on a Russian Interpol arrest warrant. Going to the police station right now," Browder wrote.
A Spanish national police spokeswoman confirmed Browder had been detained on Wednesday morning in Madrid and taken to a police station to check on the arrest warrant. Police found out there that the order was no longer valid, she said.
The spokeswoman said the financier was released minutes after his detention.
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In an image of a detention report posted by Browder himself on Twitter, police identified the reason for the detention as "fraud."
A Moscow court sentenced Browder, who was born in the US and is now based in London, to nine years in prison in absentia over tax evasion and funnelling money overseas. He has dismissed the accusations.
The financier and Ivan Cherkasov, an associate who was sentenced to eight years behind bars, were also ordered to pay four billion roubles ($A92 million) to the Russian government in damages.
Browder has advocated for US sanctions targeting Russian officials over human rights abuses. He was the driving force behind the Magnitsky Act, named after Browder's former employee Sergei Magnitsky, who died in jail after accusing Russian officials of involvement in a tax fraud scheme.
The Magnitsky Act became a sore point between the US and Russia after it was signed into law by former President Barack Obama in 2012. Shortly afterward, President Vladimir Putin signed a law banning American citizens from adopting Russian children, in a move widely viewed as retaliation.
