Fears that "several suspects" could attack Brussels are behind the decision to shut the city's Metro and put troops on the streets.
Police are hunting for others as well as Salah Abdeslam, who officers in France suspect of being the eighth terrorist involved in the Paris attacks.
Interior Minister Jan Jambon told Belgian TV station VRT: "This is several suspects, which is why we have introduced such a concentration of resources."
"We are following the situation minute by minute. There is no point in trying to hide it. There is a real threat, but we are making every effort day and night to cope with this situation."
Brussels remains at its highest terror threat level today - signifying "a serious and imminent threat" - over fears several individuals with arms and explosives could launch a Paris-style attack.
The underground transport network in the Belgian capital will remain closed until this afternoon - and dozens of events have been cancelled over concerns large crowds could make an attack more likely.
Heavily armed soldiers and police are patrolling the city, and many public places are also closed.
Belgium's prime minister, Charles Michel, is set to review the security situation with his ministers later today - and it is possible that the lockdown will be extended into the working week.
One picture circulating on social media shows a newlywed couple in the city posing for pictures on one of Brussels' cobbled streets, flanked by two armed soldiers for security.
Getting married in #Bruxelles on November 21st, 2015. This will definitely be a day to remember. pic.twitter.com/632jACQ5Wo
— Joris Marseille (@JorisMarseille) November 21, 2015
There are concerns that at least one suspect involved in the Paris attacks could be in Belgium - with a police source describing fears that Saleh Abdeslam could be "trapped and desperate" in Brussels.
Belgian media are reporting that police fear that Salah could be wearing a suicide belt he is said to have had on as he travelled from Paris to Brussels in the early hours after 129 people died on 13 November.
French TV station France Info meanwhile, said that police are hunting up to a dozen people intending to carry a similar attack to that in Paris.
A number of bars and nightclubs remained closed on the normally busy Saturday evening in the Belgian capital with one restaurant that stayed open saying that all of its 35 bookings were cancelled.
The rest of the Belgium, outside Brussels and the Vilvoorde area, remains at level three, meaning an attack is "probable".
Meanwhile, Great Britain's Davis Cup team, including Andy Murray, has delayed travelling to Belgium until Monday amid continuing security fears but are still planning to play in the final.
Belgium, and Brussels in particular, have been at the centre of investigations into the Paris attacks after it emerged that two of the suicide bombers lived in the country.
Belgian newspaper La Derniere Heure reported that special forces police had arrested four people in a "suspicious vehicle" in the Place du Grand Sablon marketplace in central Brussels on Saturday.
The Federal Prosecutors Office in Brussels denied reports that chemicals and explosives were among an arsenal found during searches on Friday night.
