Tens of thousands of separatist-minded Catalans have swarmed into Barcelona, giving raucous backing to independence advocates as they launched a campaign to elect a majority of secessionists in regional parliamentary elections this month.

The campaign for the September 27 ballot began on the Catalan National Day holiday that separatists have used for years to rally hundreds of thousands of people in the city to call for the creation of a new European nation.

As the protest kicked off, no crowd counts had been released but television images showed at least tens of thousands jamming Barcelona's broad avenues and waving pro-independence flags. Organisers hoped to attract at least 500,000 using the slogan "Let's start building a new country".

The top candidate on a list representing a block of pro-independence parties told reporters earlier that separatists wanted to stage an independence referendum so Catalonia's voters could directly express their sentiment.

But as the central government in Madrid refused, "the only way we can know what the people want is to use the elections on the 27th", said the candidate, Raul Romeva.

Pro-independence parties need 68 seats in the 135-member parliament to push forward. Polls show them on track to win a slim majority.

Independence backers say a win would give the policy makers a mandate to start drafting of a Catalan constitution and attempt secession negotiations with the central government.

A unilateral declaration of independence for the highly industrialised north-eastern region, with a population of 7.5 million and about 18 per cent of Spain's economic output, could be made by the parliament by 2017, separatists say.

"I am willing to go to the end of this process if we have the majority of seats in parliament," said Artur Mas, Catalonia's regional leader. But he said independence aspirations must be put on hold if backers fail to win a majority in the regional parliament.

The central government in Madrid has consistently insisted that independence for the region is unconstitutional. Spanish officials have also said they will not let citizens who live in Catalonia be stripped of their Spanish citizenship.