"We have enough elements to believe that the threat is specific and imminent," Belgian Foreign Affairs Minister Didier Reynders said as he arrived at an emergency National Security Council meeting on Saturday morning.
The Brussels metro was closed for the whole day, although buses continued to run, and officials recommended that all assemblies be called off, although some football match organisers insisted that they would go ahead.
Police and military presence was beefed up for fear of an attack on shopping centres, places of assembly or on public transport and a security committee for the Brussels region also met on Saturday morning.
Brussels airport remained open.
The hunt for Salah Abdeslam, who took part in the Paris attacks along with his brother, Ibrahim, continued Saturday.
Although a third body was found at the scene of Wednesday's police raid in Saint Denis, near Paris, which resulted in the death of the attacks' alleged mastermind, Abdelhamid Abaaoud, investigators appear to believe that Abdeslam has fled to Belgium.
A Belgian website has reported that he had approached a friend in Brussels, saying that the attacks had "gone too far" but that he dare not give himself up because he feared that the Islamic State (IS) armed group, which was behind the attacks, would take revenge on his family.
In related news:
- A Belgian of Moroccan origin has been arrested in the south of Turkey, according to the Dogan news agency;
- European Union interior ministers, meeting in Brussels on Friday, agreed to impose systematic controls on the borders of the Schengen no-passport zone and to introduce the Passenger Name Recognition system for air travellers;
- The French Senate endorsed the extension of the country's state of emergency for three months, making it easier to confine suspects to house arrest and ban groups judged extremist;
- Nearly 800 searches of premises, 90 arrests, 164 house arrests and 174 seizures of arms have taken place since the state of emergency was imposed;
- The first permits for funerals of the Paris attacks' victims have been issued for 112 of the deceased;
- Four of the Paris attackers have been identified as Brahim Abdeslam (31), Bilal Hadfi (20), Samy Amimour (28) and Omar Ismail Mostefai (29);
- Two of the Stade de France suicide bombers are believed to have entered Europe through Greece in October, one using a Syrian passport in the name of Mohammad al-Mahmod, the other that of a Syrian soldier, Ahmad al-Mohammad, killed several months ago;
- The UN Security Council unanimously passed a French resolution authorising "all measures necessary" to fight IS.