Thousands of people, many carrying Catalan separatist flags, demonstrated in Barcelona on Sunday after German police arrested Catalonia's former president Carles Puigdemont, according to an AFP reporter on the scene.

They chanted "Puigdemont our president" and "Freedom for the political prisoners", as they made their way from the city's main boulevard, Las Ramblas, to the offices of the European Commission in the Catalan capital.

"This Europe is shameful!," they chanted outside of the European Commission offices.

German police arrested Puigdemont earlier on Sunday on an international warrant issued by Spain as he crossed over by car from Denmark.

Spain's supreme court said Friday would prosecute for "rebellion" 13 Catalan separatists, including Puigdemont, for their role in the region's failed bid to break away from Spain.

If found guilty, they face up to 30 years in prison. Twelve more face less serious charges such as disobedience.

"What they are doing these days is totally disproportionate, we are being treated like criminals for wanting independence," Rosa Vela, a 60-year-old teacher, told AFP at the demonstration.

Puigdemont left Spain for Belgium after the Catalan parliament on October 27 declared independence.

He had hoped to gain international support from there for the independence of Catalonia, one of Spain's wealthiest regions.