Organisers said the event was the biggest climate march ever in Belgium, with 65,000 protesters braving steady drizzle, according to police.
The well-mannered crowd was filled with activists and many families, with children holding placards asking politicians to fight harder to assure a greener future.
"Politicians where are you? You have to be with us, the people. There is no planet B," Nicolai said.
The march, dubbed Claim The Climate, ended with speeches and performances at the Parc Cinquantenaire that overlooks the European Union institutions.
Public transport throughout the city was free on Sunday for the occasion and the SNCB national train service offered a cut-rate ticket of five euros.
Trains from other Belgian cities were packed with protesters, with many complaints on social media that the SNCB had not adequately anticipated the surge in travellers.
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