The yellow vest demonstration was intended to be against social injustice on the day of European Parliament elections. But it degenerated into disorder, with some protesters pelting buildings and smashing barricades. Police intervened to disperse the violent demonstrators.

Brussels police spokeswoman Ilse Van de Keere said around 350 people were briefly detained but were released later Sunday.

Police on horseback patrolled the historic centre and scuffles broke out in different areas.

The protest came as Belgians voted on Sunday at the regional, federal and European levels, dubbed a "triple election".

A far-right, anti-immigrant party made huge gains in the northern Flanders region, according to near-complete national election results. The result looks set to complicate a national coalition government.

Belgium is split along linguistic lines, with French-speaking Wallonia in the south and Dutch-speaking Flanders in the north, and governments invariably are coalitions made up of parties from both regions.

The anti-immigrant Vlaams Belang party surged to 18 seats in the 150-seat parliament, up from three in 2014. It will be the second-biggest party in Flanders behind the nationalist N-VA party, which has 25 seats, after losing five.

In bilingual Brussels, the Greens had a major surge in support.

Australian Associated Press