The US adopted the Magnitsky Act in 2012, imposing travel bans and sanctions on Russian officials seen as involved in the attorney's death. Russian President Vladimir Putin retaliated by signing a law that banned Americans from adopting children in Russia, as well as drawing up a list of US officials barred from the country.

Browder's arrest in Madrid echoes a similar move last October by Russia, which put out a notice on Interpol that bypassed the organisation's headquarters and requested cooperation from other member states to arrest him.

Browder, a British national since giving up his US citizenship in 1998, was briefly barred from traveling until Interpol notified its members to ignore the notice.

This time, the notice appears to have gotten through to Madrid police, who arrested Browder just before an 11am meeting with a Spanish prosecutor about a money-laundering case tied to fraud uncovered by Magnitsky.

"In the back of the Spanish police car going to the station on the Russian arrest warrant," Browder wrote on Twitter before his release. "They won't tell me which station."

Last year, Russian officials accused Browder of being complicit in Magnitsky's death, a claim Browder called "crazy." Russian President Vladimir Putin accused Browder of being a "serial killer." In December, a Moscow court found Browder guilty in absentia of tax evasion and sentenced him to nine years in jail as well as ordering the collection of 4.2 billion rubles ($93 million) in unpaid taxes.

In March he testified before members of Parliament in Britain saying he feared for his life after the poisoning of former Russian spy Sergei Skripal in a small English town.

"I'm at risk, at high risk," he said. "I don't spend my life living in fear, but I am definitely at risk."

The Russian Prosecutor General's Office said it hasn't yet received information about Browder's arrest, according to the state-run Tass news service.

Loading

Canada and the Baltic States have also adopted versions of the Magnitsky Act, while British legislators have backed similar measures that will allow the government to freeze assets of human-rights violators.

Browder said in October that he'd received a letter from Interpol's Commission for the Control of Files, the body responsible for vetting data, informing him of its decision to dismiss Russia's request to put him on the organization's wanted list.

"The most important thing is that they informed all member states not to entertain any notices against me going forward from Russia," Browder said in an interview.