LONDON (Reuters) - Three Catalan independence leaders being held in pre-trial detention have lodged a complaint against their imprisonment with a U.N. panel, hoping to exert pressure on Madrid to free them, lawyers said on Thursday.

FILE PHOTO: Dismissed Catalan vice president Oriol Junqueras arrives to Spain's High Court after being summoned to testify on charges of rebellion, sedition and misuse of public funds for defying the central government by holding a referendum on secession and proclaiming independence, in Madrid, Spain, November 2, 2017. REUTERS/Javier Barbancho/File Photo

The former deputy leader of the Catalan regional government and leaders of two separatist groups are accused of sedition in their bid to declare Catalonia independent from Spain.

While other figures behind last year’s referendum and independence declaration - both acts declared illegal under Spain’s constitution - fled to Brussels, Oriol Junqueras, Jordi Sanchez and Jordi Cuixart were arrested and denied bail pending their trial.

A judgment from the U.N. Working Group on Arbitrary Detention, a panel of human rights experts, would not be legally binding, but lawyers said it would be a way of applying pressure on the Spanish state.

“This case does not ask the U.N. to adjudicate on the issue of Catalan independence, but seeks the U.N’s reaffirmation that governments cannot repress political dissent through arbitrary detention,” Ben Emmerson, acting for the three, told a news conference in London.

The Catalan leaders planned other legal challenges, he said.

Spain fell sharply in a recent annual Democracy Index by the Economist Intelligence Unit, which cited Spain’s use of force in an attempt to stop the referendum going ahead and a repressive treatment of secessionist politicians.

“This kind of political oppression belongs to a bygone era in Spain’s history,” Emmerson said, referring to Spain’s dictatorial past under General Francisco Franco.

The independence drive has split loyalties in the wealthy northeastern region and caused resentment in much of the rest of Spain, as well as tarnishing the image of Spain’s four-decade-old democracy abroad.

After imposing direct rule on Catalonia in October, Spanish Prime Minister Mariano Rajoy called regional elections in December to try to defuse a crisis that threatened to split the country.

But his gamble backfired when separatist parties won a majority, giving new impetus to the independence movement, led by Carles Puigdemont, who remains a fugitive from justice in Belgium.

Rajoy’s office was not immediately available for comment on Thursday.