BARCELONA, Spain — The jailed leader of the main grassroots group pushing for Catalonia to secede from Spain resigned his post Thursday to run as a candidate in the election the Spanish government called for next month to stop the region’s push toward independence.

The Catalan National Assembly said Jordi Sanchez would be a candidate on the slate headed by Catalonia’s former president, Carles Puigdemont, who is in Belgium facing extradition back to Spain along with four members of his ousted cabinet.

Sanchez and another separatist activist were jailed on Oct. 16 while being investigated for sedition for their alleged roles in a secession-related protest that trashed police vehicles in September.

Under Spanish law, he and any other pro-independence Catalan officials or activists can run in the Dec. 21 election unless they are convicted. Puigdemont plans to be his party’s top candidate while campaigning as a fugitive in Belgium.

Another eight former members of Catalonia’s regional government have been similarly jailed in another case probing a rebellion plot.

Sanchez, 53, has led the Catalan National Assembly since 2015. The group organized rallies that attracted hundreds of thousands of supporters of having Catalonia break away from Spain. Sanchez also formed part of an informal leadership council that advised Puigdemont on achieving the region’s secession.

Spain called the December election after seizing control of Catalonia. The unprecedented action followed a vote in Catalonia’s Parliament to declare the region as a separate republic on Oct. 27 in violation of Spain’s Constitution, which deems the nation “indivisible.” Spain’s government hopes that parties opposed to secession will take back control of the chamber.

Polls indicate that Catalonia’s 7.5 million residents are evenly split over secession.

Hours after Sanchez launched his political bid, a few thousand Catalans in favor of breaking from Spain gathered in a central square in Barcelona to mark the one-month anniversary of his and the other separatist leader’s imprisonment.

Many of the protestors held up signs reading “Freedom for Political Prisoners” and some carried pro-independence flags. The crowd formed the shape of a ribbon that many secessionists wear as a symbol of their solidarity with the jailed leaders.

“Their imprisonment is a punishment for the entire country,” Agusti Alcoberro, the new leader of the National Catalan Assembly, told the crowd.