Brussels woke this morning into an atmosphere of war. Army convoys passed through the streets. Armored cars were posted at central points, outside the royal palace, and at underground railway stations. Regular public statements announced the cancellation of public events. Stores were closed on Rue Neuve, the city's main commercial artery.

"It's the first time in recorded history that the street has been closed," the merchants' association said in a statement.

"We have information about different individuals carrying out attacks with weapons and/or explosives," Belgian Prime Minister Charles Michel told a press conference during which he explained that he highest state of alert had been declared in Belgium's capital.

The information pointed to the intended targets of the terror action, the prime minister added: "Shopping malls, shopping streets, public transport and, in general, places where many people are coming together."

Open gallery view Credit: AFP

"An attack similar to that in Paris last week could take place in Brussels," Michel warned.

He asked the resident of Brussels to obey instructions, keep themselves updated on developments and "to be careful but not to panic."

The state of preparedness was raised to its highest level –4 out of 4 – after a review of the situation by Belgian National Agency for Emergency Situation. Immediately afterwards, all 69 stations of the city's underground railway were closed by order of the transport department.

In addition, tram routes that include underground sections were closed. On several routes, the order to cease activity was received while trams were carrying passengers, with loudspeakers informing passengers to "alight immediately and continue your journey on foot or by taxi."

The closure of much of the city's transport system is expected to continue until midday Sunday, when there will be another situation appraisal.

The U.S. Embassy in Brussels urged Americans to “shelter in place,” stay at home and avoid public transport. The North Atlantic Treaty Organization, whose headquarters are in Brussels, also recommended that staff and their families avoid public transit and public gatherings in Brussels. The city is also home to the European Commission and many European Union institutions.

The mayors of the 19 municipalities that make up metropolitan Brussels met in emergency session and decided – on the basis of the information provided them by state officials – to cancel most municipal activities. Municipal swimming pools were closed, as were community centers, libraries and places of entertainment.

All public performances were cancelled and requests to cancel were sent to sports organizers and the organizers of private concerts.

The Brussels tourism office contacted all hotels on Saturday afternoon, asking them to update their guests of the emergency situation and advise them to avoid all crowded public places.

The warnings had the intended effect. Few people were seen in the city's commercial areas and markets on Saturday afternoon and the streets were virtually empty. Most residents preferred to stay home. Instead, the streets were occupied by patrolling soldiers in full combat gear and police.

The police command issued an emergency order: "All uniformed officers who are not armed are forbidden from taking to the streets.

"Brussels is a dead city," blared the headline on the website of La Capitale, the local newspaper. The surrealistic scene was complemented by the first snowfall of the season, which fell in heavy snow flakes on the city.

Towards evening on Saturday, a large contingent of police stopped a car carrying six young people travelling on the highway towards Wallonia, in the south of the country. La Libre newspaper reported that the arrest took place near a gas station that has been frequented in the past by the fugitive terrorist Salah Abdeslam.

Security forces raided a house in the center of the Molenbeek, a heavily-Muslim neighborhood, on Friday night and seized weapons . Weapons were also discovered in a car parked nearby.

The police said that a "key individual" in connection with terrorist activities had been arrested in the raid. But they stressed that the raid was unconnected to the state of emergency declared on Friday night.

"We will go from address to address in Molenbeek and clear whatever we can," Interior Minister Jan Jambon said.