About 45,000 people marched past the European Union headquarters in support of Catalonia's independence. The protesters chanted "Wake up, Europe!" and waved Catalonia's red, yellow and blue Estelada separatist flag.
Catalan independence supporters take to the streets: Charlotte Chelsom-Pill from Brussels
Tens of thousands of Catalan independence supporters rallied in Brussels on Thursday, to show support for deposed regional president Carles Puigdemont and protest what they see as the European Union's failure to help them following the Spanish region's independence referendum and subsequent declaration of independence.
The sea of protesters waved Catalonia's red, yellow and blue Estelada separatist flag and banners demanding European solidarity as they marched past the European Union headquarters.
Belgian police said on Twitter that there were an estimated 45,000 protesters, more than double the 20,000 organizers expected.
Catalonia declared independence following a referendum on October 1, despite the country's Supreme Court having said the vote would be illegal. Madrid then invoked Article 155 and took control of the Catalan parliament.
While around 90 percent of participants voted for independence, the voter turnout was only 43 percent. In the Catalan parliamentary vote, which passed 70 to 10, dozens of opposition lawmakers walked out, boycotting the vote.
Puigdemont and four former ministers fled to Brussels in November. They said they wanted to bring their cause to the European level after Spain charged them with sedition and rebellion over Catalonia's independence referendum.
The Spanish government on Monday revoked a European arrest warrant for the five, but Puigdemont said he would stay in Brussels for the moment as he and his allies still faced arrest in Spain if they returned for regional polls in Catalonia that Madrid has called for December 21.
The Spanish National Court had issued the arrest warrant for Puigdemont and the four former ministers after they failed to appear in court to face questioning over the independence referendum.
On Monday, Spanish Supreme Court Judge Pablo Llarena upheld the jailing of former Catalan Vice President Oriol Junqueras and former Regional Interior Minister Joaquim Forn, as well as the leaders of two Catalan grassroots separatist groups, who had been sent to prison pending an investigation into the independence campaign.
At the same hearing, six other ex-cabinet ministers from the desposed Catalan regional parliament were released on bail, which was set at €100,000 ($118,000).