Belgian PM Charles Michel says the decision to raise the terror alert level in Brussels to the highest level was taken fearing an attack “like the one that happened in Paris” last week.
The fear was that “several individuals with arms and explosives could launch an attack… perhaps even in several places”, Mr Michel said.
Some of the attackers who killed 130 people in Paris lived in Brussels.
Leading suspect Salah Abdeslam is believed to have gone back to Belgium.
A huge manhunt is under way.
The Brussels metro is closed till Sunday and people have been told to avoid crowds.
These include shopping centres and concerts, and the authorities have also recommended that large events, including football matches, be cancelled, a statement said.
The warning for the rest of Belgium stays at a lower level, which is still at a “serious” level.
The Belgian government will review the security situation in Brussels on Sunday afternoon, Mr Michel added.