Jordi Sanchez and Jordi Cuixart are accused of having encouraged hundreds of protesters to block national police who raided the offices of the regional government on September 20, ahead of Catalonia's banned independence referendum on October 1.
Earlier on Monday, Catalonia's police chief Josep Lluis Trapero -- also charged with sedition for his handling of the pro-independence protests and for allegedly failing to stop the referendum -- was granted provisional release by the court.
Each of the three could face up to 15 years in jail if convicted.
Cuixart heads the pro-independence group Omnium Cultural, while Sanchez is the president of the Catalan National Assembly (ANC) association. Both groups have tens of thousands of members each.
The leaders are accused of calling the demonstration outside the Catalan regional economy ministry, which resulted in several Civil Guard police cars being vandalised as officers were trapped for hours inside the building.
Both the ANC and Omnium blasted the decision to detain their leaders, announcing protests.
Omnium described the decision to remand the pair in custody as "deplorable" and "intolerable in a democratic society".
"The mobilisation continues, they will not be able to imprison an entire town," it said.
For its part, the ANC called on protesters to hammer their pots and pans in a traditional "cacerolada" demonstration on Monday night, with a "protest strike" to follow at noon on Tuesday.
