The march — organized by the three main trade unions — protested the €11.3 billion ($15.3 billion) in budget cuts and austerity measures the government will enact next year to get the country's deficit within 3 percent of GDP and meet targets set by the European Union.
The unions specifically protest the extension of early retirement, cuts in unemployment benefits and the social security budget.
Union leaders later Friday met with Elio Di Rupo, the Socialist leader who pushed through the budget measures and is set to be sworn in as prime minister on Monday.
Union leaders estimated between 60,000 and 80,000 protesters marched through the streets of Brussels, choking city-center traffic which was compounded by a partial transport strike around the country.
"It shows how much the workers and the welfare recipients of this country want to have their voices heard by the coming government and say one more time that austerity is not the solution, it is the problem," Anne Demmelen, General Secretary of the FGTB socialist union said.
Beyond excessive noise from firecrackers and horns, no incidents marred the rally.
The march came on the heels of Thursday's protests in the Greek capital, and Wednesday's two-million strong demonstrations across Britain — showing a groundswell of labor discontent as the financial crisis grips the continent, forcing governments across the board to cut budgets and impose austerity measures.
On Friday, Di Rupo met with European Union President Herman Van Rompuy to explain the measures he planned to push through parliament as soon as his government would be sworn in.
Unions say the grand coalition of Socialists, Liberals and Christian Democrats seek to ride out the financial crisis on the backs of the working class.
They say that if the government does not heed their call for changes in the budget cuts, unions will consider a general strike.
With its high overall debt, Belgium is forced to make cuts, especially since its bond yields have been rising in markets.