Brussels police came under attack on Thursday after they arrested a Muslim woman who refused to remove her face veil.
Protesters threw metal barriers and trash cans at a police station in Molenbeek. The district of the Belgian capital has a high number of Muslim immigrants.
Belgium and France passed laws last year that ban the wearing of full-face veils in public.
Johan Debecker, the head of Brussels-West police authority, said the unnamed woman became aggressive during questioning.
“She head-butted our policewoman and the policewoman now has a broken nose,” he told reporters.
Some 100 people surrounded Molenbeek police station late on Thursday night, with scuffles breaking out.
Phillippe Moureaux, the town mayor, said an extreme Islamist group, Sharia4Belgium, was behind the trouble.
Sharia4Belgium issued a statement on its website on Friday, condemning what it called “a terrorist attack of Brussels’ assassination squads”.
Last month, an Antwerp court jailed the group’s spokesman Fouad Belkacem for two years for inciting racial hatred against non-Muslims.