Surging unemployment and financial disarray have stoked a fever of separatism in Catalonia, a comparatively prosperous part of Spain whose 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 central government said the crowd was 600,000 strong. Catalan 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-years-old Teresa Cabanes, who came from Santa Coloma de Gramanet, in the outskirts of Barcelona, to march.
"We feel that the central government is fooling with us. We Catalans are giving away a lot of money to Spain."
No to Europe
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 march ended after nightfall without any incident and no people were arrested, police said.
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.
The region's president, Artur Mas, has suggested he could seek independence if he is not given more control over tax raised from Catalonia.
"If we cannot reach a financial agreement, the road to freedom for Catalonia is open," he repeated on Tuesday. Mas did not attend the march but said he backed it in spirit.
The annual Diada holiday is typically commemorated with a fiesta in the Catalan capital with song, dance and a floral offering to Rafael Casanova, a hero of the siege.
But the outpouring on Tuesday was a sign that the economic crisis has transformed issues of cultural identity into a mainstream political movement bent on autonomy. A poll by the regional government in July showed for the first time that more than half of Catalonia's population favours independence.
For Mas and his nationalist Convergence and Union party (CiU), that translates into demands for control over tax.
Catalonia has long sought more autonomy, though without a violent separatist movement like the Basque Country's ETA.
Economists calculate Catalans pay at least €12 billion more in taxes per year to Madrid than they receive back for services like schools and hospitals. Many Catalans say the figure - difficult to calculate because of a complex system of transfers - is even higher, up to €16 billion.
"A lot of people who were not into independence are more and more into it now," said Elvira Farre, a retired secretary from Barcelona. "They are being driven into it by their feelings but also by their wallets."
Government worker Jauma Turra said: "It's like a marriage you can't put up with any more."
The Madrid government took a dim view of what leaders there saw as Mas's stirring up Catalans ahead of the march.
"Catalonia has serious deficit and employment problems and this is not the moment for messing around or disputes or controversy," Prime Minister Mariano Rajoy of the conservative People's Party said on Monday.