Former Catalan leader Carles Puigdemont, who is accused of helping to organise an illegal independence referendum, has been arrested in Italy, his lawyer said.
Mr Puigdemont is being held under a European arrest warrant issued by Spain in 2019.
His lawyer, Gonzalo Boye, said he was detained on the island of Sardinia.
Mr Puigdemont, 58, who now holds a seat in the European Parliament, is wanted by Spain for his role in a failed attempt at secession four years ago.
Earlier this year, he and two of his associates lost their immunity from prosecution.
Sardinian media reported earlier in the week that he was due to attend an event in Alghero on Sunday, so his presence on the Mediterranean island was expected.
It was also reported that he was invited by a Sardinian pro-separatist group.
Mr Puigdemont’s office said that he had travelled to Alghero from Brussels to attend a folklore festival.
He was also scheduled to meet with the regional head of Sardinia and its ombudsman.
After his arrest, he was transferred to a jail in Sassari, a city about 40 kilometres north-east of Alghero, the Italian news agency ANSA reported.
Earlier reports said a judge in Sassari would issue a ruling on Friday on whether Mr Puigdemont should be freed.
He and a number of his separatist colleagues fled to Belgium in October 2017, fearing arrest after holding an independence referendum for Catalonia that the Spanish courts and government said was illegal.