Bill Browder tweeted that he had been held in Madrid on a “Russian Interpol arrest warrant”.
He also posted an image of the warrant and another picture taken in the back of a police car.
Mr Browder later said he had been released after the Interpol general secretary in Lyon advised Spanish police “not to honour the new Russian Interpol red notice”.
It is the “sixth time Russia has abused Interpol in my case”, he added.
Mr Browder has spent more than a decade trying to expose what he calls the “looting” of his investments in Russia by corrupt officials.
A lawyer he hired to investigate what happened to his money, Sergei Magnitsky, died in 2009 in Russian custody.
It is believed Mr Magnitsky may have been murdered.
.@Billbrowder has been arrested in Spain. I interviewed him in London last week. He never holds back on his criticism of Mr Putin. Russia requested an Interpol arrest warrant a few months ago for charges linked to corruption and murder all of which he denies. pic.twitter.com/ymrAShXryQ
— Mark Stone (@Stone_SkyNews) May 30, 2018
Mr Browder tweeted that “ironically”, the reason he was in Madrid was to “give evidence to senior Spanish anti-Russian mafia prosecutor Jose Grinda about the huge amount of money from the Magnitsky case that flowed to Spain”.
“Now that I’m released my mission carries on. Meeting with Prosector Grinda now,” he added.
Mr Browder has written a book called Red Notice: How I Became Putin’s No.1 Enemy.
Deeply concerned to learn that the Russians have tried to get Bill Browder arrested in Spain. He is a British citizen and we will establish the full facts ASAP.
— Sir Alan Duncan MP (@AlanDuncanMP) May 30, 2018
Minister of state at the Foreign Office, Sir Alan Duncan, said he was “deeply concerned to learn that the Russians have tried to get Bill Browder arrested in Spain”.
“He is a British citizen and we will establish the full facts ASAP,” he added.
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Foreign Secretary Boris Johnson tweeted that he had “just spoken to Bill Browder – very glad that he has now been released”.
Mr Johnson said “Moscow should concentrate on bringing those responsible for the murder of Mr Magnitsky to justice”.