The Belgian government said Saturday it had concrete evidence of a planned terrorist attack that would have employed weapons and explosives.
The revelation came after officials shut down the underground railway system in Brussels and raised the city’s terrorism threat to the maximum of level 4.
The symbolic Atomium monument remained closed, while a music festival and various football matches – including the first league match between top teams KSC Lokeren and RSC Anderlecht – were cancelled.
The city of Brussels asked all bars, bistros, cafes and dance clubs to close at 6 pm (1700 GMT) as a precaution, but not restaurants, Belga news agency city a spokesman for the mayor as saying.
Police picked up four people in the city centre, Belga reported.
Prime Minister Charles Michel cited an immediate threat as grounds for elevating the warning level, according to a Belga report.
Michel said the Belgian government had prepared additional security measures for the capital but did not provide details.
“We have sufficient information to gauge that the danger is clear and immediate,” Foreign Minister Didier Reynders said.
“The threat is sufficiently high to raise the terror alert level to level 4,” Interior Minister Jan Jambon added.
At an emergency meeting of the national security council on Saturday morning in Brussels, government officials met with police and security forces to discuss additional measures.
Belgium’s emergency centre on Saturday declared a level 4 threat – the highest possible – overnight for the capital. No details were released at the time, other than to say the situation had received a “new assessment” in the wake of the Paris bombings.
Service was halted on Brussels’ underground rail network Saturday and is due to be out of service until 3 pm (1400 GMT) Sunday as a precautionary measure, according to the website of transport operator Stib.
Buses, trams and trains continued to provide service in Brussels, with authorities intensifying their checks on passengers.
Extra security was also reported at the nation’s airports and train stations.
“The recommendations for the population are very simple,” said a crisis centre spokesman.
“We ask you to avoid places with large gatherings of people in the Brussels capital region, such as concerts, popular events, train stations, airports and public transportation.”
In neighbouring Luxembourg a hotel was cleared in the centre of the capital, but the all clear was given later in the afternoon. One arrest was made.
The German Foreign Office urged visitors to Brussels to be cautious and avoid large crowds in the region.
Locations and events to be avoided included concerts, railway stations, airports, public transport and shopping centres, the office said.
Many of the participants in the November 13 Paris terrorist attacks had ties to the Belgian capital. The death toll from those attacks rose to 130 On Friday, French media reported. Brussels is also headquarters to the European Union and the NATO defence alliance.
Outside of Brussels, the rest of the country was put on level 3, which means “a possible and likely” threat.