British businessman Bill Browder, a well-known critic of the Kremlin and anti-corruption advocate, was arrested in Madrid and briefly held on a Russian warrant.
Browder - the principal champion of the Magnitsky Act, a 2012 US law that imposed wide-ranging sanctions on certain Russian officials - confirmed via Twitter that he was released after about an hour in custody.
Spanish police acknowledged the warrant was not valid. But the arrest in a European capital raised concerns about Russian influence on foreign soil.
That question is of particular concern in Britain, where a former Russian spy and his daughter, Sergei and Yulia Skripal, were targeted in a nerve-agent attack in March.
British Foreign Secretary Boris Johnson said he spoke to Browder and was pleased to see him released, but added: "Moscow should concentrate on bringing those responsible for the murder of Magnitsky to justice."