A Spanish judge has jailed two key members of the Catalan independence movement.
Jordi Sánchez and Jordi Cuixart, who lead prominent separatist groups, are being held without bail while they are under investigation for sedition.
The men were leading figures in the 1 October independence vote, which the Madrid government regards as illegal.
A protest over the detentions has begun in Barcelona and other demonstrations across Catalonia are expected later.
Mr Sánchez, who heads the Catalan National Assembly (ANC), a pro-independence organisation, and Mr Cuixart, leader of Omnium Cultural, appeared before the High Court in Madrid on Monday.
They are being investigated over a protest on 20 September in which a crowd blocked Civil Guard officers inside a building in Barcelona, Catalonia's regional capital.
How the Spanish press sees it
El Periódico (Barcelona, unionist): "The 'process' had been having an incredibly quiet few days. That was until the news of the detention without bail for the presidents of the two main pro-independence civil society organisation, Jordi Sànchez and Jordi Cuixart, which has shaken up the landscape in a way that should have been registered by seismographs."
Ara (Barcelona, pro-independence):"With separatists and the Spanish government looking to see who moves the next piece, the National Court turned over the table yesterday".
La Vanguardia (Barcelona, moderate pro-independence): "The decision of the magistrate adds a new, inopportune and worrying element of tension to the conflict that opposes the Catalan government and the Spanish government. Now it will be more difficult to reach agreement through dialogue."
El Mundo (Madrid, unionist): "The rule of law continues its struggle against secessionist claims, which suffered important blows yesterday."
Following the referendum, Catalan leader Carles Puigdemont signed a declaration of independence, but halted its implementation to allow negotiations.
He has called for talks to take place over the next two months.
However, the Spanish government has warned that Catalonia must revoke the declaration or face direct rule from Madrid.
Mr Puigdemont has also angered Madrid by refusing to clarify whether or not he declared independence last week.
Mr Puigdemont, who has been given until Thursday to clarify his position, hit out at the government on Twitter following news of Mr Sánchez and Mr Cuixart's detention.
"Spain jails Catalonia's civil society leaders for organising peaceful demonstrations. Sadly, we have political prisoners again," he wrote.
In a video recorded before his court appearance and released on his Twitter account after his detention, Mr Cuixart instructs separatists to "never lose hope because the people of Catalonia have earned their future".
Pro-independence supporters have called for further protests, demanding the men's release.
Just hours before the ruling, the High Court freed the head of Catalonia's police force, Josep Lluis Trapero.
His force, the Mossos d'Esquadra, is accused of failing to help Spain's Guardia Civil police tackle thousands of pro-independence protesters in Barcelona during the run-up to the referendum.
Prosecutors had called for him to be held in detention.