Shops and schools remain closed in a move that has divided locals, with some concerned about the impact on the local economy.
Belgian police have arrested 16 people during operations following the Paris attacks.
But Brussels resident Salah Abdelslam, Europe's most wanted man following the attacks, continues to elude capture.
At its highest terrorism-threat level, Brussels is a city quickly resembling a ghost town.
But authorities say they will do whatever is necessary to keep their citizens safe after receiving details of an allegedly imminent Paris-style terrorist plot.
Weapons and explosive devices have been uncovered following raids on Brussels properties.
Prime Minister Charles Michel says the decision to keep many of the city's facilities closed was made during a Security Cabinet meeting.
"What we fear is an attack similar to what happened in Paris, with several individuals (who could) even perhaps launch offensive weapons in several places at the same time, which leads us to think that the potential targets are those which are highly concentrated in population, such as shopping centres, commercial centres, public transport."
Belgium remains on edge as Brussels resident Salah Abdelslam continues to evade authorities.
He has become Europe's most wanted man since authorities formed the belief that he was behind the coordinated wave of attacks in Paris just over a week ago.
Prime Minister Michel maintains the cancellation of major events in the capital is necessary and is keeping the subways and schools closed for at least a third day.
He says the decision is based on information from the country's crisis centre.
"The analysis from (the crisis centre) has led us to identify the threat more precisely public areas, commercial streets and commercial centres, demonstrations and lively areas and transport. The Brussels region is now at level 4, and the rest at level 3."
A third person was charged in connection with the Paris attacks.
To avoid hindering the investigation, Belgium's interior minister, Jan Jambon, says he will not disclose any more details about the threat detected.
"The question was, 'Why did we go to the level of threat to 4?' And the Prime Minister said there are indications that people with arms and explosions are in the region of Brussels. But we don't give any more details because there are a lot of investigations going on and we don't want to make errors against the procedures, so we don't communicate details on this."
Shopping centres and stores have been forced to close, and many people are staying indoors.
The extraordinary precautions have caught some Belgians by surprise just three weeks from Christmas.
A Brussels resident named Juana says she supports the government's decision.
"Well, it's not good for the economy, but it's good for security because at least the terrorists, the baddies, won't come to the shops -- like here, for example. It's good like this."
But a local trader named Mohammed, whose business is among the many suffering a big economic blow, says he believes the fear being generated is what the terrorists want.
"No, this is giving the advantage to this so-called terrorism, because people are stuck at home and do nothing. Nothing. This morning, they cancelled all the football matches. Why? For nothing. The shops, for nothing. We are doing the terrorists a favour. It's like, 'Look, everybody is at home, the shops are closed.' No, we have to go on living."
Amid the presence of armed soldiers on the streets, some stores were open over the weekend.
But with so few people around, most shut their doors at midday.
The city's museums have also been closed, and the crisis centre has advised the public to avoid large gatherings.
The government says it will reassess the terrorism-threat level after the additional 24 hours.
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