Tens of thousands of Belgian protesters took to the streets of Brussels on Wednesday to rally against austerity measures introduced by the government of Prime Minister Charles Michel, police and trade unions said.

Brussels police said 81,000 demonstrators turned out while the main union organising the protest put the number at 100,000, making it one of the biggest rallies of its kind in Belgium in recent years.

The demonstrations in the capital were peaceful so far, but Belgian television reported isolated clashes between dockers from the Flemish-speaking northern port city of Antwerp and the security forces.

Some unions went on strike on Wednesday and a national rail strike is planned for Friday.

The protests are the biggest in Belgium since November 2014, when riot police fired tear gas and water cannon in violent clashes during an anti-austerity demonstration by at least 100,000 people.