Reuters journalists saw about a dozen emergency vehicles, including police, outside the Islamic and Cultural Centre of Belgium, a large Saudi-established institution including a mosque situated 200 metres (yards) from the European Commission.
A spokeswoman for the fire service said it had taken a call from the mosque from a person saying they believed that they had found anthrax powder, prompting the deployment of specialist crews. There was no immediate word on what the substance was.
Belgian media said envelopes of white powder were found.
Since the Paris attacks on November 13 by French and Belgian Islamic State militants, there have been reports of threats against mosques used by Belgium's half million Muslims, among them some from an unknown group calling itself Christian State.