Belgian authorities closed down the subway system in Brussels today, and flooded the streets with armed police and soldiers in response to what they said was a threat of Paris-style attacks.

The decision to raise the threat alert to the highest level in the Belgian capital came as the manhunt continued for suspected militant Salah Abdeslam, 26, who slipped back home to Brussels from Paris shortly after the carnage in neighbouring France just over a week ago.

A relative calm descended on the city centre on Saturday, reported CBC's Adrienne Arsenault, who's in Brussels.

"It doesn't look like any other Saturday night in downtown Brussels. All of the stores along this main street have been closed all day long. The malls have been closed. The metro has been closed. Concerts, theatres, sporting events have been cancelled. There's a huge military presence in this city. There are soldiers stationed at the major hotels."

Owners of bars and restaurants were told by officials that they can keep businesses open, but those that do would be defying advice from the mayor to close for the night, said Arsenault. 

Belgium's Prime Minister Charles Michel said the decision to raise the alert was taken "based on quite precise information about the risk of an attack like the one that happened in Paris."

'Serious and imminent' attack

The tip authorities received suggested that an attack would involve "several individuals with arms and explosives launching actions, perhaps even in several places at the same time," he said.

Local authorities have warned the public to avoid crowds because of a "serious and imminent" threat of an attack.

The U.S. Embassy in Belgium urged Americans there "to shelter in place and remain at home," while the U.S. European Command issued a 72-hour travel restriction for U.S. military personnel on travel to Brussels – a city of more than one million that is home to the headquarters of the European Union, the NATO alliance and offices of many multinational corporations.

On Brussels's central square, the Grand Place, tourists snapped selfies Saturday night as a green army truck full of soldiers pulled up next to a lit Christmas tree. 

The Belgian federal prosecutor's office said several weapons were discovered during the search of the home of one of three people arrested in connection with the Paris attacks on Nov. 13, but said no explosives were found.

Authorities across Europe, the Middle East and in Washington are trying to determine how a network of primarily French and Belgian attackers with links to Islamic extremists in Syria plotted and carried out the deadliest violence in France in decades – and how many may still be on the run. 

A new potential link emerged Saturday in Turkey, where authorities said they detained a 26-year-old Belgian suspected of connections to Islamic extremists – and possibly to the Paris attacks. The private Dogan news agency identified him as Ahmet Dahmani, and said he is suspected of having explored areas in Paris that were targeted in the attacks.

Brussels on highest alert due to 'imminent' threat2:19

Also on Saturday, Belgium's military could be seen protecting the Radisson Blu in Brussels. Several people were killed during an attack on the Radisson Blu in the Mali capital Bamako on Friday.

Brussels on highest alert

Belgian soldiers stand guard outside the Radisson Blu hotel in central Brussels after officials announced a 'serious and imminent' threat of an attack Saturday. (Youssef Boudlal/Reuters)

Abdeslam is believed to have crossed back into Belgium from Paris shortly after the attacks, in which his elder brother Brahim blew himself up at a café. 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 centre said weekend games in Belgium's two professional divisions should now be postponed, but most outside Brussels appeared set to go ahead.

Abdeslam, along with Abdelhamid Abaaoud, the presumed organizer of the Paris attacks who was killed in a raid Wednesday just outside the French capital, grew up in the Molenbeek district of Brussels.

Canadians warned to be cautious 

Also Saturday, the government of Canada advised travellers in Belgium to exercise a high degree of caution because of the threat of terrorism.

That comes even though the Belgian government hasn't expanded the Brussels security lockdown to a nationwide security alert.

In light of events of France and Belgium, the 15 members of the UN Security Council quickly and unanimously passed a resolution on the Islamic State of Iraq and Syria (ISIS) on Friday.

UN vote on ISIS1:37

The resolution, sponsored by France, calls on UN countries to join the fight against the Islamist militants and calls ISIS an "unprecedented" threat to international peace and security.

The resolution called on member states to take "all necessary measures" to defeat ISIS.

"Today we send a clear, unambiguous message that there will be no respite from our collective efforts to stop, suppress and destroy ISIL," said Matthew Rycroft, the U.K. ambassador to the United Nations, who also holds the rotating Security Council presidency.

Until 2006, Belgium had a permissive gun law by European standards, and many weapons used in the 1990s Balkan wars easily found their way into the Belgian criminal underworld. At the same time, the Justice Ministry was hurt by austerity measures, rendering it powerless to dig into the root causes of the problem.

"It is relatively easy to get your hands on heavy arms in Brussels," said Brice De Ruyver, a professor of criminology at Ghent University, who was security adviser to the prime minister from 2000 to 2008. "That applies to terror and serious crime. That is because the illegal arms trade has been neglected far too long. And once you have a reputation, it is tough to get rid of it."

Gunmen behind last January's attacks on a Paris supermarket and the office of the Charlie Hebdo magazine were believed to have purchased weapons from Belgian arms dealers.

With files from Reuters and CBC News