Exiled Catalan separatist leader Carles Puigdemont has been detained in Italy on a Spanish arrest warrant.
Spain has accused him of sedition for a breakaway independence referendum in 2017 that the courts ruled illegal.
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.
Mr 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.
After Mr Puigdemont and two ministerial colleagues fled, Spain jailed nine other Catalan leaders for sedition for their role in the breakaway vote. They were pardoned by Spanish Prime Minister Pedro Sánchez in June.
Madrid failed in its court bid to extradite Mr Puigdemont and two ministerial colleagues and they were later elected to the European Parliament in 2019.
As MEP the Catalan ex-leader initially had immunity from prosecution but the Parliament voted to strip him of that last March. He appealed against the decision but in an initial ruling at the end of July the EU’s general court said there was no immediate risk of arrest.
There are at least two question facing the Italian judiciary: the validity of the Spanish extradition request, and the powers of Italian judges when the case is yet to receive a ruling by the EU’s top court.
Spanish authorities say the European arrest warrant was “never deactivated”. And Mr Puigdemont’s lawyers say that no court within the EU can execute a European arrest warrant until a final ruling has been made.
Catalonia’s new president Pere Aragones – also a separatist – condemned what he called the “persecution” of Mr Puigdemont.
Mr Puigdemont’s arrest is uncomfortable for the Spanish government, says the BBC’s Guy Hedgecoe in Madrid. The left-wing coalition in Madrid had just started engaging in talks with the Catalan government in an attempt to calm tensions and find a long-term solution to the crisis.
The Spanish government said in a statement: “The arrest of Mr Puigdemont corresponds to an ongoing judicial procedure that applies to any EU citizen who has to answer to the courts.”
He should “submit to the action of justice like any other citizen”, it added.
Catalonia is one of Spain’s richest, most distinctive regions and has enjoyed a high degree of autonomy. But many Catalans feel they pay more to Madrid than they get back, and there are historical grievances too, in particular Catalonia’s treatment under the dictatorship of General Franco.