Hundreds of thousands of Catalans rallied in Barcelona on Friday to show support for independence from Spain.
Local police said 1.4 million people turned out but the Spanish government put the figure at no more than 550,000.
A separatist coalition of parties is leading the polls ahead of Catalonia's regional elections this month.
The Together For Yes campaign says if it wins a majority in the regional assembly it will push through plans for an independent Catalan state.
But the Spanish government has promised to block any move to break away. Madrid said an unofficial Catalan referendum last November was invalid.
The November poll saw 80% of voters back independence but turnout was just 40%.
Together for Yes kicked off its campaign late on Thursday night ahead of the rally on Friday, when Catalans celebrate their national day.
The coalition comprises political groups from both the left and right, who have come together in the face of continued Madrid government opposition to a referendum.
One of those marching on Friday said it was a "definitive" year.
"The goal is to become independent, because we're suffocating - economically and emotionally," Silvia Palomares told Reuters.
Polls suggests a majority of Catalans favour a referendum on independence but are evenly divided over whether they want to secede.
The Spanish government says it will block any unilateral declaration of independence, reports the BBC's Tom Burridge in Barcelona.
Such a move would be unprecedented in the history of the European Union, our correspondent adds.