Officers say the man may be hiding within a building.
Armed “specialised” police and helicopters are at the scene, in the west of the Belgian capital.
Belgian police have sealed off part of the suburb and live television images show armed officers hiding behind buildings.
RTL were told by Forest mayor Marc-Jean Ghyssels police “suspect the presence of an armed man”.
A witness told RTBF that police told them not to leave their homes.
Magistrate Marc-Jean Ghyssels told RT he believed there was one armed men, or several men, in a flat on rue Jean-Baptise Vanpé on the outskirts of Brussels.
According to a witness an armed man was screaming in the street, but the magistrate could not confirm this.
A spokesman for the Bruxelles-Midi police said officers were called to the Forest area at 1pm local time. A Polish man in shock told a shopkeeper that he had seen a murder in the area earlier today.
Police went to the address he told them, but found nothing there.
Mr Ghysseis added he did not think anyone has been injured.
He added: “At this stage, it seems to be domestic dispute.
“There is nothing to indicate it has anyhting to do with terrorism.”
The Forest area was the scene of a shootout between terror police hunting for the and Paris attack suspect Salah Abdeslam.
The Belgian-born of Moroccan descent was thought to be staying in a flat there and when police raided the presumed empty flat they were ambushed by his accomplice Mohamed Belkaid.
He fired explosives at police to distract from a rooftop getaway of another accomplice. In the November 2015 Paris attacks 130 people were killed in a series of strikes including the Bataclan theatre.
A police spokeswoman denied media reports that an armed gunman was hiding in the area. She said that the special units of the federal police had been deployed after a Polish man reported that someone had been murdered in the morning.
She said: ”They are searching the buildings but we can say there has not been any terrorist attack.”
Around 350 students at a nearby school, École Sainte-Alène de Forest, were hurried inside from their lunch break upon hearing the news.
The headmaster told RTL the children would remain in the school.
More to follow…