The Catalan government on Tuesday issued an urgent plea for help to the European Union after two independence leaders were jailed as part of a sedition investigation, as protesters took to the streets to demand the "political prisoners" be freed.

Jordi Turull, spokesman for the Catalan government, said the imprisonment of the two men over September protests in Barcelona was "an attack on fundamental rights" that the European Union had a duty to protect.

Jordi Sanchez, head of the Catalan National Assembly, and Jordi Cuixart of Omnium Cultural, were remanded in custody without the possibility of bail on Monday night by a National Court judge investigating alleged crimes of sedition, which carries a prison term of up to 15 years.

They are being investigated for their role in a tense night of protest in Barcelona on September 20, as national police carried out sweeping raids and arrested several high-ranking Catalan officials in a bid to halt the banned referendum.

As night fell in Barcelona, thousands of protesters descended on the city's famous boulevard, the Passeig de Gracia, many of them carrying candles and chanting "Freedom for political prisoners", "Out, forces of occupation!" and "Not one step back!"

Protests were also held in other towns and cities across Catalonia as well as in Valencia, Mallorca and Madrid.