A Britain-based financier who has spearheaded a U.S. law targeting Russian officials over human rights abuses says he has been arrested in Spain at Russia's request.

William Browder tweeted about the arrest Wednesday, 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.

Browder wrote: "Urgent: Just was arrested by Spanish police in Madrid on a Russian Interpol arrest warrant. Going to the police station right now."

A Spanish national police spokesman said the arrest couldn't be immediately confirmed by authorities. The spokesman couldn't be named in line with internal police rules.