Former Catalan president Carles Puigdemont was detained in northern Germany and is set to appear court on Monday. Spain is seeking to have Puigdemont extradited to face charges of sedition, rebellion and embezzlement, following his role in Catalonia's independence fight.
Puigdemont had entered Germany from Denmark after on Friday leaving Finland, where he had attended a conference.
A demonstration in Barcelona against his arrest by tens of thousands of Catalans tipped over into clashes on Sunday night.
Outside the central government offices in the Catalan capital, riot police beat flag-waving protesters back with batons, leaving several with blood streaming down their foreheads. Three protesters were arrested and 50 suffered minor injuries, police said.
Spain's Supreme Court ruled on Friday that 25 Catalan leaders, including Puigdemont, would be tried for rebellion, embezzlement or disobeying the state relating a referendum held in Catalonia last October which called for the wealthy northeastern region to separate from Spain.
The Madrid government deemed the referendum, which was widely boycotted by opponents of independence, illegal and took over direct rule of Catalonia following a symbolic declaration of independence by the Catalan parliament.
The court has also reactivated international arrest warrants for four other politicians who went into self-imposed exile last year. Puigdemont and fellow separatists have all denied any wrongdoing.
It ordered five separatist leaders, including the latest candidate to become regional president, Jordi Turull, to be jailed pending their trial.
The Madrid government's forceful legal moves deal a potentially fatal blow to the independence movement and signal an end to one of the country's worst political crises since the return of democracy four decades ago, although separatist sympathies are likely to simmer.
Days if not weeks
Puigdemont will appear before a regional court in the northern German town of Neumuenster where a judge is expected to decide whether he will remain in custody.
Another court, the Higher Regional Court in the town of Schleswig, will be responsible for deciding whether to grant Spain's extradition request. German prosecutors have said the whole legal process, including possible appeals, could take several days if not weeks.
Puigdemont, who has been living in Brussels since leaving Spain, does not plan to apply for political asylum in Germany, his lawyer Jaume Alonso-Cuevillas told Catalan radio on Monday.
The 55-year-old former journalist is unlikely to find much support among German politicians who have largely backed Spanish Prime Minister Mariano Rajoy's crackdown on Catalan separatism.
In comments published on Monday, Elmar Brok, a German member of the European Parliament and close ally of Chancellor Angela Merkel's conservatives, said that Puigdemont had "clearly violated Spanish law and the Spanish constitution."
"He would be well advised to end the matter peacefully," Brok told regional newspaper Neue Osnabruecker Zeitung.
Brok said that the quest of the Catalans for independence was hopeless and that Madrid and Barcelona should instead start negotiations about an improved autonomous status.
German Justice Minister Katarina Barley told ARD broadcaster on Sunday the extradition question would be handled by regional courts and she saw no need for Berlin to intervene for now.
European governments, some of whom face separatist movements of their own, have generally been supportive of the Spanish government.
In Edinburgh, the Scottish government, which advocates independence from the United Kingdom, said it would cooperate with Madrid over a request to extradite former Catalan education minister Clara Ponsati.
Scottish First Minister Nicola Sturgeon said that it nevertheless supported "the right of the people of Catalonia to determine their own future."
The drawn-out crisis has also hit the regional economy and caused a business flight. But S&P Global said its upgrade for Spain reflected a positive outlook for the economy and budgetary consolidation.
It was the second upgrade for Spain into single-A territory this year. Spanish bond yields were at close to 15-month lows on Monday morning.
By Angus Berwick and Michael Nienaber/Reuters
Reporting by Angus Berwick and Emma Pinedo in Madrid; and Michael Nienaber in Berlin.