Spanish police said as many as 1.8m Catalans packed the streets of Barcelona to demand the right to vote on a potential split from Spain.
Those calling for independence have seen their ambitions boosted by the independence referendum scheduled for next week in Scotland.
The protesters were dressed in red and yellow, the colours of the Catalan flag, and formed a "V" for "vote" in a show of support for holding a referendum.
In a festive atmosphere on Catalonia's national day, La Diada, people stood side by side along two of Barcelona's main arteries to form the huge "V".
Many wore T-shirts saying "Ara es l'hora" ("Now is the time") in Catalan.
"We want a say in politics and our future. We've won back our sovereignty (by getting independence on the political agenda) and realised the strength we have, if we mobilise, to change things," said Carme Forcadell, head of the National Catalan Assembly, one of the organisers of the event.
Catalonia is a wealthy region in Spain's north-east with its own language and culture.
Its long-standing independence movement has grown significantly over the last decade, exacerbated by Spain's economic crisis and what many see as a deaf-ears tactic by the Spanish government in Madrid.
The Catalan regional government, which like other Spanish regions has a large degree of autonomy, has called a non-binding referendum on 9 November over whether Catalonia should become a separate state.
The Madrid government says the vote is illegal and cannot go ahead and the issue is deeply entrenched, pitting Catalonia against the rest of Spain.
"We don't expect anything good from the Spanish government. All we get is misunderstanding, intolerance, threats and totally anti-democratic attitudes. They've always been like that," said one of the protesters.
"We just want to be treated equally, with respect. Nothing more. We are Catalans, not Spaniards."
Polls show that 80% of Catalans want to vote on the issue.
Catalonia's campaign has drawn momentum from the upcoming referendum on whether Scotland should split from the United Kingdom.
The fact that Scots have been allowed to vote at all was singled out as the main motivation for taking part in today's event, organisers and participants said.
"We don't understand why that is constantly denied. We look up to Scotland," said another protester.
"They are so lucky to belong to a country that allows that kind of vote."
An official Catalan opinion poll shows support for an independent state has tripled to 45.2% in March 2014 from 13.9% in March 2006.
Many commentators in Madrid, where there is fierce opposition to Catalan nationalism, question the way such polls are carried out.
Spain's state pollster CIS found this year that around 45% of Catalans support full independence, 20% would support a federal state and 23% back an autonomous region.