Hundreds of thousands of Catalans took 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 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 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.
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 percent of Spain's population but 20 percent of its economy.
With Spain's economy in freefall from the Euro zone debt crisis, Catalans complain they are paying billions of Euros more in taxes than they receive back from Madrid, even as their regional government has been forced to fire workers and cut services.
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.
Economists calculate Catalans pay at least 12 billion Euros 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.
The Madrid government took a dim view of what they 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.