The Catalan National Assembly has called for protests across the region
A Spanish court has ordered the leaders of two powerful separatist organisations in Catalonia to be detained - a move that could cause tensions to worsen further.
Jordi Cuixart and Jordi Sanchez - presidents of the pro-independence groups Omnium Cultural and the Catalan National Assembly - are being kept in jail on sedition charges.
Protests broke out in Barcelona on Monday as news of their imprisonment spread.
They are accused of stirring up major demonstrations in the run-up to the controversial independence referendum on 1 October.
About 40% of 7.5 million Catalans voted in the referendum, with 90% of those backing independence from Spain.
"The state is playing at provocation," said Catalan government spokesman Jordi Turull after the National Court ruling.
Secretary general of Omnium Cultural, Jordi Bosch, said: "I'm calling for Catalan society to peacefully protest."
The Catalan National Assembly has called for protests across the region, with a strike planned for Tuesday.
Calls for demonstrations also spread on social media, with some planned in Catalonia and beyond, including London.
Hundreds of independence supporters gathered outside Catalan government offices, singing the region's hymn and holding signs that read "freedom for political prisoners".
A video recorded by Cuixart before his detention was sent out overnight, in which he said: "If you're watching this video, it's because the state has decided to deny me my freedom."
The activist added that his organisation would work "underground" and peacefully to further their cause.
The latest detentions came as the Spanish High Court ruled that Catalan police chief Josep Lluis Trapero would not be held in custody while an investigation continues.
He had also been arrested on charges of sedition.
Prosecutors allege he failed to rescue officers from the Civil Guard, the national police force, who were trapped by protesters inside a Catalan government building in September.
On Monday, Catalan President Carles Puigdemont failed to provide a yes or no answer on whether he would declare independence.
Madrid wanted an answer by 10am on Monday, but instead Mr Puigdemont wrote a letter to Spanish Prime Minister Mariano Rajoy calling for a meeting "as soon as possible".
Mr Rajoy had warned that Madrid will suspend Catalonia's autonomy if independence was declared.
"You still have time to answer clearly and simply," Mr Rajoy told Mr Puigdemont, warning that weeks of instability were damaging the Spanish economy.
Spain has now given him until 10am local time on Thursday to clarify his position.