Police said some 5,500 people rallied in Brussels' European quarter housing a number of EU institutions. Some held aloft banners bearing slogans including "Our people first" and "We have had enough, close the borders."
Police stepped in to disperse the crowd after projectiles and firecrackers were thrown, an AFP journalist on the scene reported.
Many marchers were supporters of the Vlaams Belang party, which opposes last week's signing of the UN accord in the Moroccan city of Marrakesh.
The agreement is designed to frame an effective international approach to migration -- a deeply divisive
...politicians call things
divisivewhen when the other side sez something they don't like. Their own statements are never
divisive,they're
principled...
issue across Europe
...also known as Moslem Lebensraum...and beyond.
A counter-demonstration drew around 1,000 people to another Brussels district, according to police.
Belgian authorities initially banned both rallies fearing unrest but the country's supreme administrative court the Council of State overturned the decision.
More than 150 states last Monday approved the pact, a Global Compact for Safe, Orderly and Regular Migration.
But NGOs have voiced doubts about its implementation both on the ground and the high seas.
Pope Francis on Sunday voiced his support for the agreement and urged the international community to show "responsibility, solidarity and compassion" in dealing with