Financier Bill Browder, a high-profile critic of Russian President Vladimir Putin, has been briefly arrested in Spain on a Russian arrest warrant.
After an hour in custody he was released because the warrant was no longer valid, Spanish police said.
Mr Browder earlier tweeted that he had been arrested on a Russian Interpol arrest warrant.
Interpol rejected the claim, saying: “There is not, and never has been, a Red Notice for Mr Bill Browder.”
The agency said in a statement: “Mr Browder is not wanted via Interpol channels.”
Interpol had previously refused to enforce a 2013 warrant for the investor, deeming it “predominantly political in nature”.
The US-born financier, who is a British citizen, tweeted a copy of an arrest document from the Spanish interior ministry, which said he was accused of fraud.
“Spanish National Police just released me after Interpol General Secretary in Lyon advised them not to honor the new Russian Interpol Red Notice,” Mr Browder tweeted.
“This is the sixth time that Russia has abused Interpol in my case.”
It is not yet clear how the confusion which led to the arrest arose.
Mr Browder is widely credited with the creation of the Magnitsky Act – a 2012 range of sanctions from the United States on top Russian officials accused of corruption.
The act was named after his former lawyer, Sergei Magnitsky, who uncovered an alleged tax fraud in Moscow – and died in a Russian prison in 2009.
Since then, Mr Browder has campaigned for investigations into the alleged fraud.
He was put on trial in absentia in 2013 for tax evasion – in the same trial where the deceased Magnitsky faced charges, which was labelled “absurd” by observers.
Mr Browder was once one of the biggest foreign investors in Russia, controlling Hermitage Capital Management.
Russian prosecutors accused him of creating a firm for tax fraud purposes. Mr Browder has always said that the charges against him are politically motivated.
British Foreign Minister Boris Johnson said he spoke to Mr Browder and was “very glad that he has now been released”.
“Moscow should concentrate on bringing those responsible for the murder of #Magnitsky to justice,” he tweeted.
Mr Browder was in Madrid to give evidence to a Spanish prosecutor about money from the Magnitsky case that was moved to Spain.
He said the arrest while on such a trip was ironic, but added: “Now that I’m released my mission carries on.”