Exiled Catalan separatist leader Carles Puigdemont has been detained in Italy on a Spanish arrest warrant.
At the time he was president of Catalonia but fled to Belgium after the vote. He lives there and sits as a member of the European Parliament.
Police were apparently waiting for him at Sardinia's Alghero airport and he is due in court in the coming hours.
Puigdemont had travelled to the Mediterranean island for a Catalan folklore festival, his lawyer said.
A judge in Sardinia will have to decide whether he should be released or extradited.
The 2017 referendum prompted Spain's deepest political crisis for decades, with the Catalan regional parliament declaring independence, and Madrid then imposing direct rule over the region.