Belgium has been without a government since elections in June, as Dutch- and French-speaking political parties remain split over autonomy plans.
The trade unions say economic and social policy had been neglected because of the political paralysis.
They were also protesting against high food and fuel prices.
At least 18,000 people had taken part in the protest march in Belgium's capital, police said.
The demonstration was organised by the three main trade unions, which represent some three million of Belgium's population of nearly 11 million.
"People are fed up with the discussions between the political parties. This is a very serious warning to the parties to take care of the people," Bernard Noel, one of the leaders of the CGSLB trade union, told Reuters.
"If they don't listen today, I am afraid of a backlash against politicians and things could get tougher in coming weeks," Mr Noel added.
Many protesters also said they wanted to keep Belgium's social security united, rejecting demands by Flemish parties in the richer Dutch-speaking regions to split it.
Unlike Belgium's linguistically-divided politicians, the country's trade function on a federal basis.
Their protest follows a warning by the European Commission that the prolonged political crisis was beginning to hit Belgium's economy.
Belgium has been run by a caretaker government under outgoing Prime Minister Guy Verhofstadt since the elections, and the cabinet has been unable to take policy decisions.
On Monday Belgium's King Albert asked Mr Verhofstadt to urgently form a new interim government.
Güncelleme Tarihi: 16 Aralık 2007, 10:56