Thousands of protesters have filled the streets of Barcelona after Spain's High Court jailed five Catalan politicians who played a role in the region's declaration of independence.

Demonstrators on Friday evening carried the single-star flag of the presumptive Catalan Republic and signs with slogans such as "Freedom for Political Prisoners".

Catalonia declared independence from Spain in October 2017 after a political back-and-forth between the regional and national governments.

The declaration went unrecognised by the international community.

Spanish Judge Pablo Llarena ordered detention without bail for Jordi Turull, the president of Catalonia's regional government, former regional legislators Josep Rull, Raul Romeva and Dolors Bassa, as well as the ex-president of the parliament, Carme Forcadell.

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Llarena said an investigation into their pro-secession moves "has reflected clear insurrection".

The High court issued international arrest warrants for six more Catalan politicians.

Several Catalan politicians have fled Spain to avoid being put in jail, including former Catalan President Carles Puigdemont, who fled to Belgium in October.

Marta Rovira, the head of the Catalan Republican Left party who was also summoned to the High Court, announced Friday she had left Spain to live in exile in Switzerland.

Oriol Junqueras, the former Catalan vice president, has been in prison since December.

Clashes amid an uncertain future

Social media posts showed demonstrators clashing with Catalan police, who are called "Mossos".

The Mossos were attempting to keep demonstrators separated from the Delegation of the Spanish Government in downtown Barcelona.

Demonstrators reported injuries. Al Jazeera was unable to verify how many were injured or the severity of the wounds.

Càrregues i ferits pic.twitter.com/zU9vpzqtrF — Jordi Borràs (@jordiborras) March 23, 2018

As clashes erupted, Catalonia's political future remained uncertain.

Catalonia held snap elections in December called by Spanish Prime Minister Mariano Rajoy.

Secessionist parties won that election, a repeat of the 2015 poll that sent Catalonia on the path to its declaration of independence.

The separatist majority failed to elect a regional government in a Thursday vote. If they cannot agree to a government in two months, another regional election will be held.

Rajoy said on Friday he is "not in favour of [Catalonia] having repeat elections. It's not good."

The prime minister said the Catalan people have voted and it's up to politicians to settle their differences within the confines of Spanish law, which does not allow for secession.