Despite the police blockade at the entrances, the organisers initially allowed the speeches and discussions to continue.
According to the internet portal "Catholic Herald", the planned appearance by German Cardinal Gerhard Ludwig Müller was also able to take place.
The cardinal is said to have been shocked by the police presence.
"This is like Nazi Germany," Müller said according to a blog post by author Rod Dreher.
According to the conference website, participants included Brexit supporter Nigel Farage, Hungarian Prime Minister Viktor Orban, former French presidential candidate Eric Zemmour, Gloria von Thurn und Taxis and former German head of the Office for the Protection of the Constitution Hans-Georg Maaßen.
The National Conservatism Conference is organised by a conservative think tank. The aim of the conference is to bring together "public figures, journalists, academics and students".
According to the organisers, around 600 participants were expected to attend the event.
"Extreme right is not welcome"
The mayor of the Saint-Josse district of Brussels, Emir Kir, confirmed on the platform "X" (formerly Twitter) that he had issued the order in order to "ensure public safety".
According to the BBC, Kir is said to have said in a message to the organisers that some participants at the conference held racist and homophobic views.
"Among these personalities are several participants, particularly from the right-wing conservative, religious right and the European far right," his statement reads. And: "The extreme right is not welcome".
Belgian Prime Minister Alexander De Croo also commented on the conference on the "X" platform.
Such a dissolution is unacceptable, as the "ban on political assemblies is unconstitutional", said De Croo.
"Municipal autonomy is a cornerstone of our democracy, but it can never override the Belgian constitution, which has guaranteed freedom of expression and the right to peaceful assembly since 1830," concluded De Croo.