Wearing a mask became compulsory on Wednesday for 1.2 million people living in the Brussels region, as Belgium battles one of the most serious coronavirus outbreaks in Europe.
Many of the protesters did not wear masks, but carried placards saying "Corona circus” or "It’s my body, it’s my choice”.
"It is not the virologists and doctors who are going to dictate the rules in our country,” a protester who gave his name only as Michel told AFP. "We are being treated as children.”
The call to demonstrate was launched on social media by a group of citizens under the slogan "Viruswaanzin”, or "viral madness” in Dutch, a movement that first appeared in The Netherlands.
The group has demanded the resignation of Flemish virologist Marc Van Ranst, one of the most prominent members of the panel of experts consulted by the Belgian government on the pandemic, and who is also in the sights of the Flemish extreme right for his left-wing stance on social issues.
Sunday’s demonstration drew conspiracy theorists and anti-vaccination activists.
One protester wore an orange life jacket with the words "Ready for the second wave”, while another had written on his shirt "No to this masquerade”.
Belgium has the highest per capita death rate from COVID-19 in the world and infections are again rising after earlier success in bringing the disease under control.
The number of cases in the country of around 11.5 million people neared 78,000 on Sunday. Nearly 10,000 deaths have been registered.