The European quarter of the city saw thousands of protesters demanding that transatlantic trade talks between the US and EU be halted.
The demonstration on Tuesday follows recent comments by senior government ministers in Germany and France saying that the protracted TTIP negotiations are "effectively dead" and should end.
However, supporters of TTIP immediately responded by mounting a robust defence of the deal.
BusinessEurope, the Brussels-based lobby organisation representing the business community, said EU trade policy is "very important" and had brought "large benefits" to EU companies and citizens.
It urged member states to "do their utmost" to ensure a rapid signature and implementation of the CETA agreement with Canada and to "make all the efforts possible" to conclude an "ambitious and comprehensive" TTIP with the United States.
Emma Marcegaglia, the President of BusinessEurope, said, "The US is the EU's largest trade and investment partner and we are in the middle of a complex bilateral negotiation.
"Both sides knew from the start that it would not be easy for the two leading world economies to agree on an ambitious and forward looking trade and investment agreement. But they also knew that the expected benefits would be worth the efforts."
She added, "TTIP is too important to fail. We are in the course of a hard negotiation and we need strong political commitment from the EU and the US to reach an agreement. Business on both sides is strongly engaged and we expect politicians to deliver. We need to boost growth and jobs in Europe and TTIP is one of the best means to achieve it."
Her message was reinforced at a BusinessEurope briefing on Wednesday where the group's Director General, Markus Beyrer, also spoke out in defence of TTIP and the risk of what he calls "rising protectionism."
The briefing heard that trade is a "key driver" of economic growth and both TTIP and CETA could help boost both the EU and world economies.
In Tuesday's protest, however, trade unions, human rights and farming groups took to the streets in the Belgian capital out of concerns against the trade deal.
Greenpeace Europe spokesperson Mark Brady said, "We are against these trade deals because they are a threat for the environment, for health, for labour regulations, and they give so much power to multinational corporations."
Organisers claim there were between 10,000 and 15,000 protestors, but Brussels police estimated 9,000 protesters.
There is a continuing wave of opposition to CETA and TTIP in Europe.
On Saturday, massive demonstrations took place across Europe against the free trade agreements, particularly in Germany. Opponents of CETA and TTIP say these agreements would hurt food and labour standards, as well as the environment.
European Commission spokesperson Daniel Rosario said, "The Commission is fully aware of the lively debate taking place across member states about trade policy."