Anti-virus software creator John McAfee has been detained at a Spanish airport after being indicted in the US on fraud charges, according to the Reuters news agency.
McAfee, who designed software of the same name, was reportedly arrested in Barcelona on Saturday while boarding a flight to Istanbul using a British passport.
Reuters says he has now been moved to Madrid, where he will face an extradition process to the US.
Prosecutors in the US have unsealed an indictment against the software mogul over charges that he wilfully failed to file tax returns, and that he evaded taxes.
McAfee, 75, is a famous libertarian who believes that tax is wrong and has in the past boasted of not paying his taxes.
He launched a presidential campaign in the US in 2018, but failed to gain any traction as an independent candidate or as one for the Libertarian Party.
The Securities and Exchange Commission revealed it has brought civil charges against McAfee, claiming he made more than $ 23.1m (£17.8m) in undisclosed compensation from cryptocurrency recommendations, consultancy and speaking engagements.
He has also sold the rights to his life story for a documentary.
McAfee is a staunch advocate of cryptocurrency use, and has frequently promoted them, including using it as a basis for his presidential run.