The 11 September holiday marks the fall of Barcelona in the Spanish War of Succession in 1714 and has become a focus point for the region's secessionist movement.
Around 600,000 supporters of Catalan independence rallied in Barcelona on the region's main holiday on Wednesday, according to an estimate from the city's police force.
The day marks the fall of Barcelona in the Spanish War of Succession in 1714. Since 2012, it has become the date of massive rallies for the region's secessionist movement.
Quim Torra, President of the Government of Catalonia, said: "I think this huge demonstration, this huge concentration of thousands and thousands of Catalans, all of us are here for a very clear message. It's a very understandable message that our objective for the following weeks and following months is the independence of Catalonia. I think that this is a clear message to Europe and to the world."
The rally comes weeks ahead of the highly anticipated verdict from the Supreme Court in a case against 12 separatist leaders, on charges including rebellion. The defendants face several years behind bars if found guilty, and a heavy punishment would most likely spark public protests in Catalonia. The verdict is expected this month or next.
The movement, however, is going through its most difficult period since separatist sentiment was fuelled by the previous decade's economic difficulties, from which Spain has only recovered in recent years.
Polls and the most recent election results show that the region's 7.5 million residents are roughly equally split between those in favour and those against breaking with the rest of Spain.
Rally attendee Jordi Terol said: "There are many people still doubtful and they don't really know which way to go. They are Spanish and Catalan and they prefer to stay within Spain.
"Like myself, I am Spanish and Catalan but the situation made me lean towards the Catalan side because of what is happening. But I think if the jailed political leaders get strong unfavourable sentences, it will make more people join our cause."