Dressed in the colours of the country's main trade unions, protesters waved flags and held banners saying, "People first, profit last", "Don't touch my Volkswagen", and "Victims of Wolfsburg", in reference to the German city where the carmaker is based.
Car plant employees from Germany, France and as far away as Portugal joined the rally.
Employees of companies in other sectors joined the rally to express their solidarity.
But after talks with Guy Verhofstadt, the Belgian prime minister, the company said on Friday that up to 3,000 workers could produce the latest Audi model at the Brussels plant from 2009.
Trade union vow
Trade union officials addressing the crowd said they were not assured by the promise and would continue to fight to get the best deal for the employees.
Erwin Declerck, of the ABVV trade union, said: "For the past decade, VW employees have been asked to be enormously flexible. But it now turns out this flexibility has not brought us anything."
The Belgian plant, which has been affected by labour action in the past few weeks, produces 204,000 vehicles a year, 190,000 of which are Golf models.