Thousands of Catalans are thronging the streets of Barcelona to mark their national day - the "Diada" - and show support for an independence referendum.
Catalonia's pro-independence government plans to hold the referendum on 1 October, defying Spain which deems it illegal. Catalonia has passed a law to secede from Spain if the vote is Yes.
Opinion polls suggest the vote, if it takes place, will be very close.
With their own language and customs, Catalans already have much autonomy.
But there is a widespread feeling in the region - one of Spain's richest - that too much of its tax revenue goes to Madrid.
Spain's economic woes since the 2008 financial crisis - including chronic unemployment - have fuelled the pro-independence mood in Catalonia.
In recent days Spain's Guardia Civil police have raided several Catalan printing shops suspected of preparing material for the referendum.
The crowd in central Barcelona is expected to swell to as many as 500,000, as Catalans of all ages descend on the city. Nearly 2,000 buses were chartered to bring people to the rally.
The crowd - many sporting T-shirts in the national colours - is to form a giant cross, the regional daily La Vanguardia reports.
The 11 September Diada marks the fall of Barcelona in the War of Spanish Succession in 1714 - a defeat for Catalan forces.
Last week Spain's Constitutional Court placed a legal block on the independence referendum. A majority of Catalans want the vote to go ahead, polls suggest, to settle the thorny issue of independence.
The left-wing mayor of Barcelona, Ada Colau, cautiously backs the referendum, but has asked the Catalan parliament for guarantees to make the vote as inclusive as possible.
In November 2014, Catalonia held an unofficial "consultation" on independence - and some 80% of those who voted backed it.
But turnout then was relatively low and the vote was non-binding, as the Constitutional Court had ruled it illegal.