A well-known British financier turned anti-Kremlin activist who used to be the boss of late anti-corruption Russian lawyer Sergei Magnitsky, said Wednesday he was detained in Spain at Moscows request.
"Just was arrested by Spanish police in Madrid on a Russian Interpol arrest warrant. Going to the police station right now," William Browder tweeted.
Browder also posted a photo of himself in the back of a police van but a police spokesman could not immediately confirm the arrest.
The US-born British citizen has led a campaign in memory of his former employee Magnitsky.
Magnitsky went public with details of massive fraud by Russian state officials before being charged with tax evasion and later dying in detention after spending 11 months in squalid prisons in 2009.
A Moscow court in December sentenced Browder in absentia to nine years in a penal colony after convicting him of deliberate bankruptcy.
His campaign for Magnitsky has had strong resonance worldwide.
In 2012, the US passed the "Sergei Magnitsky Act" which imposed a visa ban and froze the assets of Russian officials implicated in the lawyer's death.
The act became a symbol of prison abuses in Russia and strained ties with Washington.
Then earlier this month, British lawmakers backed a measure to impose sanctions against people guilty of human rights violations in memory of Magnitsky.