Police fired water cannons and tear gas Sunday in Brussels to disperse stone-throwing protesters near the European Commission's headquarters after tens of thousands of demonstrators marched through the city against COVID-19 vaccinations and restrictions that aim to curb the fast-spreading omicron variant and protests turned violent.

Although Belgium announced a slight easing of coronavirus restrictions on Friday, despite record infections, the government also said people must have booster shots after five months to maintain the COVID-19 passes which allow them to access bars or cinemas.

The passes, which are mandatory for anyone wishing to enter a restaurant, museum or many other public spaces, have sparked fierce opposition among some Belgians.

In scenes reminiscent of similar clashes last November, when around 35,000 protesters took part in demonstrations that started peacefully but later turned violent, some streets in the Belgian capital were soon filled with acrid tear gas.

The building housing the European diplomatic service and a sandwich shop were broken into, a Reuters witness said.

Some protesters let off fireworks as police advanced in force into a nearby park where large groups of demonstrators had gathered, the water cannon surrounded by officers wearing body armor, helmets and carrying riot shields.

One demonstrator, standing on a stage, told the others to put up their hands, saying the protesters would not give up.

"I'm angry about the blackmail that the government is doing, mostly for the young people but for everybody, but mostly for the young people, they are really blackmailing that everybody has to go for the vaccine," Caroline van Landuyt, who said she had been vaccinated against COVID-19 herself, said.

Belgium is in the midst of the fifth wave of COVID-19 infections, with the peak not expected for at least a couple of weeks. Some 89% of adults in Belgium are fully vaccinated and 67% have now also received a booster shot.

"I was very angry that my children had to do have the vaccine. They want to travel, they want to do sports competitions, and they can't do it without a vaccine, but they did not want to, it's just blackmail," she added.