Belgian riot police have fired tear gas and water cannon during clashes with demonstrators while an estimated 100,000 people marched through Brussels in the first mass protests against the new government's austerity measures.

Demonstrators overturned cars and threw paving stones and fireworks during Thursday's protest, which opposes economic reforms announced by Prime Minister Charles Michel's centre-right coalition.

Riot police armed with clubs and shields charged the rowdiest groups of demonstrators, who also set rubbish bins on fire and made makeshift barricades.

Paramedics treated at least one protester, who suffered a broken nose.

The rally, which started peacefully but turned violent, is the first in a series of measures planned by Belgian trade unions including strikes in several provinces in coming weeks, followed by a general strike throughout the kingdom on December 15.

Police said there were at least 100,000 demonstrators in Brussels while the unions said up to 130,000 protesters showed up.

Rail companies had sold low-cost tickets to increase protest numbers in the capital.

Public transport services were restricted because of the protests, partially paralysing the capital of the European Union on a day that finance ministers from countries that use the euro were gathering there.

Violent protests

Member states like Greece, Spain and Italy have also experienced violent protests against austerity.

The cabinet planned to hold a crisis meeting with the three main unions later on Thursday.

Workers and staff at steel firms, the ports of Antwerp and Zeebrugge, the post office and in education were also planning to stage a work slowdown.

Belgium has a long postwar tradition of collective bargaining between employers and workers, and successive coalition governments representing a full scale of public opinion often have been able to contain social disagreements.

But the current coalition, made up of three pro-business parties and the centrist Christian Democrats, is the first in decades that has been able to set such a clear free-market agenda.

Belgian unions as well as the socialist, green and extreme left parties oppose a decision to scrap plans for the usual automatic cost-of-living raises next year.

They also reject plans to raise the retirement age from 65 to 66 from 2025 and to 67 in 2030.

The last large nationwide demonstrations against austerity were held in February 2013 attracted up to 40,000 people.

The new government is likely to face the same problems as before, most notably the unavoidable division between Belgium's Flemish-speaking north, which tends to be more conservative, and a French-speaking, more liberal south.