Violence broke out at a protest against anti-covid measures in Brussels on Sunday, where police said tens of thousands of people were participating.
The march began peacefully but police later fired water cannon and tear gas in response to a group of participants throwing projectiles, an AFP photographer witnessed.
Several of the demonstrators caught up in the clash wore hoods and carried Flemish nationalist flags.
The stand-off with riot police took place near the Belgian capital's EU and government district.
Police said 35,000 protesters marched from the North Station in Brussels against a fresh round of COVID measures announced by the government on Wednesday.
The demonstration, called "Together for Freedom", largely focused on a ban on the unvaccinated from venues such as restaurants and bars.
Europe is battling another wave of infections and several countries have tightened curbs despite high levels of vaccination, especially in the west of the continent.
Belgium, one of the countries hit the hardest by the latest wave, on Wednesday expanded its work-from-home rules and strengthened curbs against the unvaccinated.
