Hundreds of thousands of Catalans have taken to the streets of Barcelona in an unprecedented show of mass support for autonomy from Madrid, blaming Spain's economic crisis for dragging their wealthy region down.

Surging unemployment and financial disarray have stoked calls for separatism in Catalonia, a comparatively prosperous part of Spain whose political leaders say their wealth is being sucked dry by the central government.

Crowds waved red and yellow striped Catalan flags - one of the oldest still in use in Europe - and sang the Catalan anthem on a national day marking the conquest of Catalonia by Spain's King Philip V in 1714 after a 13-month siege of Barcelona.

The regional government said the crowd was 600,000 strong. Local police gave figures as high as 1.5 million.

Marchers said the sheer size of the crowd - swollen with people from around the region who descended on its capital in bright sunshine - would at last make Madrid hear their message.

"This is a blow for the government. People like me came from everywhere. I don't think they were expecting something as big," said 53-year-old Teresa Cabanes, who came from Santa Coloma de Gramanet in the outskirts of Barcelona.

"We feel that the central government is fooling with us. We Catalans are giving away a lot of money to Spain."

They held up banners and signs saying "No to the Fourth Reich", "No to Europe", "Independence Now!" and "Catalonia: the New European State".

The huge volume of people overwhelmed the mobile phone network, which shut down for hours under the strain.

Marchers who had attended Catalan national day rallies for decades, including others that attracted hundreds of thousands, said this was the biggest they could recall.

The show of anger and ethnic pride will play into the hands of regional authorities, who are trying to force the central government to yield control over taxes raised in Catalonia.

Catalans speak a language similar to, but distinct from, the Castillian Spanish spoken in the rest of Spain.

The region accounts for 15 per cent of Spain's population but 20 per cent of its economy.

Reuters