Separatists have won a clear majority of seats in Catalonia's parliament, according an exit poll after an election that could set the region on a collision course with Spain's central government over independence.

The main secessionist group "Junts pel Si" (Together for Yes) would get between 63 and 66 seats in the 135-strong assembly, while smaller leftist party CUP would secure another 11 to 13 seats, according to the poll released by local broadcaster TV3, the largest carried out.

They would jointly obtain 49.8 percent of the vote, amid an expected record turnout, in what would be a big boost to a secession campaign which has been losing support over the last two years.

"Junts pel Si" and CUP had said before the vote that such a result would allow them to unilaterally declare independence within 18 months, under a plan that would see the new Catalan authorities approving their own constitution and building institutions like an army, central bank and judicial system.

The Spanish centre-right government of Prime Minister Mariano Rajoy, which has opposed attempts to hold a referendum on secession, has called the breakaway plan "a nonsense" and vowed to block it in court. Spain's constitution does not allow any region to break away.

Catalan secession remains highly hypothetical, but the strong pro-independence showing is a blow for Rajoy less than three months before a countrywide election.

It also creates additional uncertainty over potential talks over a more favourable tax regime and laws that better protect language and culture, which analysts say are needed to soothe Catalan discontent.

Many of the 5.5 million voters had said on Sunday that they did not believe Catalonia would become independent and had used their ballot as a way to press the Catalan and Spanish authorities to discuss those issues.