Catalan separatist leader Carles Puigdemont was detained on Thursday by police on the Italian island of Sardinia.
Puigdemont's office said he was in the city of Alghero to attend the Adifolk International Exhibition when he was stopped by border police.
The former Catalan government head has been the subject of a European arrest warrant issued by Spain.
Madrid wants him extradited to face charges of sedition, claiming he helped organise a 2017 independence referendum deemed illegal by Spanish courts.
As an elected member of the European Parliament since 2019, Puigdemont had enjoyed immunity from prosecution and had been living in self-imposed exile in Belgium.
However, in March this year the European Parliament stripped him of that immunity.
Puigdemont's lawyer confirmed his detention in a tweet, adding that it was made on the basis of a European arrest warrant from 2019, which he said had previously been suspended.
Catalonia's 2017 referendum brought on Spain's biggest political crisis in decades and was followed by a unilateral declaration of independence by the Catalan parliament.
That prompted the central government to impose direct rule from Madrid and authorities to arrest separatist leaders.
But the issue hasn't gone away.
Earlier this month, thousands of people turned out in Barcelona to call once again for independence.