MADRID — Hundreds of thousands of separatist-minded Catalans rallied in Barcelona on Sunday to show their support for breaking away from Spain, leaving the country without its powerful and prosperous northeastern region.

Barcelona police estimated on their Twitter account that about 540,000 people attended the rally in support of a legally-binding referendum that would achieve an independent Catalonia.

Catalan separatist leader Carles Puigdemont said that he plans to propose a government-approved binding independence referendum to secede from Spain by next year. Spain, which opposes secession, argues that an independent Catalonia would be ejected from the European Union and left out from using the euro currency.

Catalonia held a non-binding vote in 2014, when around 1.6 million people voted in favor of independence. Most of the region’s 5.4 million eligible voters didn’t participate after Spain’s Constitutional Court ordered the suspension of the ballot.

In June, a Catalan judge recommended former regional president Artur Mas stand trial for staging the vote and ignoring the suspension. Mas claims the vote was carried out by volunteers.

Catalan National Day has long been used to mobilize the masses in support of secession from Spain.

Polls show most Catalans support a referendum on independence, but are roughly divided over splitting from Spain.

Catalonia shares cultural traits with the rest of Spain, but many Catalans feel their customs, especially their language, set them aside from the rest of Spain.

Acting Prime Minister Mariano Rajoy, his conservative Popular Party and two more of Spain’s main political parties oppose a Catalonian state. Only the far-left Podemos supports allowing Catalonia to hold an independence referendum.

The economically-powerful Catalonia has a thriving population of 7.5 million and accounts for 18 percent of Spain’s economic output.