Jordi S?nchez and Jordi Cuixart, who led prominent separatist groups, were being held without bail and under investigation for sedition, the BBC reported.
The men were leading figures in the October 1 independence vote regarded by the Madrid government as illegal.
Their detention led to protests overnight with more expected across Catalonia on Tuesday. S?nchez heads the Catalan National Assembly (ANC), a pro-independence organisation, and Cuixart is the leader of Omnium Cultural association.
Both the leaders appeared before the High Court in Madrid on Monday. They were being investigated over a protest on September 20 in which a crowd blocked Civil Guard officers inside a building in Barcelona, Catalonia's regional capital.
Following the referendum, Catalan leader Carles Puigdemont signed a declaration of independence but halted its implementation to allow negotiations.
He called for talks to take place over the next two months. However, the Spanish government warned that Catalonia must revoke the declaration or face direct rule from Madrid.
Puigdemont also angered Madrid by refusing to clarify whether or not he declared independence last week.
Puigdemont, who was given until Thursday to clarify his position, hit out at the government on Twitter following news of S?nchez and 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, Cuixart instructed separatists to "never lose hope because the people of Catalonia have earned their future".
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, was 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.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
Doraiah Chowdary Vundavally is a Software engineer at VTech . He is the news editor of SocialNews.XYZ and Freelance writer-contributes Telugu and English Columns on Films, Politics, and Gossips. He is the primary contributor for South Cinema Section of SocialNews.XYZ. His mission is to help to develop SocialNews.XYZ into a News website that has no bias or judgement towards any.
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