A week after the Paris bombings and shootings carried out by Islamic State militants, of whom one suspect from Brussels is at large and said by police to be highly dangerous, Brussels was placed on the top level "four" in the government's threat scale after a meeting of top ministers, police and security services.
Soldiers were on guard in parts of Brussels, including at the institutions of the European Union headquartered in the city. Brussels is also home to the headquarters of NATO.
"The result of relatively precise information pointed to the risk of an attack along the lines of what took take place in Paris," Prime Minister Charles Michel told a news conference on Saturday. The Paris carnage left 130 people dead.
The US Embassy in Belgium has urged Americans in the country "to shelter in place and remain at home" after the imminent threat warning was issue.
In a statement on its website Saturday, the embassy said "if you must go out, avoid large crowds." It said it would provide updated information as it becomes available.
"We are talking about the threat that several individuals with arms and explosives would launch an attack perhaps in several locations at the same time," Michel said.
He declined to elaborate, but said the government would review the situation on Sunday afternoon.
The metro system is to remain closed until then, in line with recommendation of the government's crisis center. Major shopping centers and stores did open on Saturday morning, with soldiers deployed outside shops. However, many began closing their doors from around midday.
The crisis center advised the public to avoid places where a lot of people come, such as shopping centers, concerts, sports events or public transport hubs. The city's museums were shut and concert venues cancelled planned evening events.
The agency has called on local authorities to cancel large events and postpone soccer matches, as well as stepping up the military and police presence.
Foreign Minister Didier Reynders said that 1,000 troops were now available for patrols, double the level of a week earlier.
Fugitive suspected militant Salah Abdeslam, 26, slipped back home to Brussels from Paris shortly after the attacks, in which his elder brother Brahim blew himself up at a cafe.
Fears of the risk Salah Abdeslam still poses prompted the cancellation last week of an international friendly soccer match in Brussels against Spain. The crisis center said weekend games in Belgium's two professional divisions should now be postponed, but most outside Brussels appeared set to go ahead.
Belguim at the Heart of Paris Terror ProbeThe alert level for all of Belgium was raised following the Paris attacks to level three out of four, implying a "possible or probable" threat. Previously, only certain sites, such as the US embassy, were at level three.
Belgium, and its capital in particular, have been at the heart of investigations into the Paris attacks - which included suicide bombers targeting a France-Germany soccer match - after the links to Brussels emerged. Three people detained in Brussels are facing terrorism charges.
Federal prosecutors said on Saturday that weapons had been found at the home of a person charged on Friday.
EU interior and justice ministers in Brussels on Friday pledged solidarity with France in the wake of the Paris attacks and agreed a series of new measures on surveillance, border checks and gun control.
French authorities have said the attacks were planned in Brussels by a local man, Abdelhamid Abaaoud, 28, who fought for Islamic State in Syria and was killed in a police siege of an apartment in the Paris suburb of St. Denis on Wednesday.
Salah Abdeslam, who was from the same Brussels neighbourhood of Molenbeek as Abaaoud and is said by officials to have known Abaaoud in prison, was pulled over three times by French police but not arrested as he was driven back to Brussels early last Saturday by two of the men now in custody.
As well as Abdeslam's brother, a second man from Molenbeek, Bilal Hadfi, was also among the Paris suicide bombers.
Belgian Interior Minister Jan Jambon told reporters he wanted a register of everyone living in Molenbeek because it was not clear at present who was there, with authorities conducting door-to-door checks of every house.
"The local administration should knock on every door and ask who really lives there," Jambon said.
The last time any part of Belgium was put on maximum alert was in May 2014 when an Islamist gunman shot dead four people at the Jewish Museum in Brussels. At that time, Jewish schools, synagogues and other institutions were put on level four.
The capital as a whole was last at the level four for about a month at the end of 2007 and the start of 2008, when authorities intercepted a plot to free convicted Tunisian Nizar Trabelsi. Brussels' traditional New Year fireworks display was cancelled.
Trabelsi was sentenced in Belgium in 2003 to 10 years for attempting to blow up a Belgian military base that houses US soldiers. He was extradited to the United States in 2013.
Reuters and the Associated Press contributed to this report.