“A parcel was found at the entrance to the mosque and found to contain white powder. We are taking all preventative measures in case it is anthrax but it is a precautionary measure,” said fire services spokeswoman Anne Wibin, who was at the scene.
The mosque, the city’s biggest, is located just a few blocks from the major EU institutions and many embassies.
Belgium has been under a heavy security clampdown in the wake of the Paris attacks on November 13 with two Belgium-linked suspects still on the run.
Wibin said eleven people, including two police offers, were being decontaminated as precautionary measure.
“There is absolutely no danger to anyone,” she said.
A worshipper said the development was worrying given the tensions after the Paris attacks.
“I’ve been speaking to the staff on the telephone and was told the imam found the package,” said Mohamed Dahmichi, standing across the street.
“It’s not good. With everything else happening in Brussels at the moment I’m afraid to leave the house these days.
“There was the letter at the mosque in Molenbeek too and now this,” he added.
Earlier this week several mosques in the Molenbeek neighbourhood, seen as a hotbed of jihadist activity inspired by the Islamic State group, received threatening letters from a group calling itself “Christian state”.