“The threat is considered serious and imminent to Bruxelles and a serious level for the whole country.”
He added: “Indications lead us to believe that the potential targets are popular locations.”
He added: “We have taken several new measures to protect public transport. There will be an increased police and military presence which can be heightened if needed. Our objective is to return to life as normal as quickly possible.”
He said the threat would be re-evaluated at another meeting this afternoon.
Security forces are hunting for Salah Abdeslam, who is thought to have played a key role in the Paris attacks and has so far escaped capture. It has been claimed that he was driven to Brussels shortly after the attacks, a week ago, possibly still wearing the suicide vest which he failed to detonate in Paris.
But officials suggested that more than one suspect is being sought in Belgium.
Belgian interior minister Jan Jambon said that the threat facing Brussels would not necessarily disappear if Abdeslam was found. “Unfortunately, the threat is wider than this [one] figure,” he said.
Bernard Clerfayt, the mayor of the Brussels municipality of Schaerbeek, revealed that authorities were hunting for two suspects.
He said: “There are two terrorists in the Brussels region that could commit very dangerous acts. As long as this threat is present, we must be very attentive.”
Armed forces patrolled the streets of Brussels, where only a few people chose to ignore advice.
“Everything, bar one defiant waffle stand, is closed,” wrote one Twitter user. “Streets quiet, armed forces all over the place.”
Elsewhere in Belgium, tourists described the atmosphere as “quiet, but fairly normal” as most restaurants and bars remained open.
Meanwhile, Abdeslam’s brother has appealed to the suspect to turn himself in, claiming that he believed he may have had a change of heart and decided intentionally not to detonate his suicide vest.
Mohamed Abdeslam told Belgian media that he would rather see his “brother in prison than in a cemetery”.
Mohamed’s other brother, Brahim, was one of the suicide bombers in the Paris attacks that killed 130 people.
He said he had noticed a change in his brothers’ behaviour about six months ago, when they gave up drinking and began going to the mosque occasionally, but said there was never any sign of radicalisation.
Asked if he had a message for his brother, he said: “Surrender.”
French police yesterday issued a new appeal to identify the third man involved in the attacks at the national stadium on 13 November.
Police posted a photo of the man on Twitter, appealing to the public for information that would help identify him. The man was among three people who died in the attacks outside the stadium.
British travellers were warned by the Foreign and Commonwealth Office to avoid “places where there is a high concentration of people” in Brussels, including stations.
However, people with tickets booked on the Eurostar between London and Brussels yesterday expressed anger that they could only exchange their tickets for a later date, rather than be granted a refund. Passengers wishing to cancel tickets, including those booked for later in the year, were subject to “normal conditions”, Eurostar said.
One man, @UnionBerlinMan, booked to travel to the UK from Brussels in December wrote on Twitter: “Normal conditions will apply? The city is in lockdown. You should help citizens & business people get back to UK quickly/safely.”
In the Turkish coastal city of Antalya, authorities have detained a 26-year-old Belgian citizen suspected of being connected to Islamic extremists and possibly to the Paris attacks – raising hopes of a possible breakthrough in the investigation.
Six British-Pakistani men travelling in old ambulances were stopped by police near a Belgian petrol station previously frequented by Salah Abdeslam yesterday afternoon. No weapons were found.