The Belgian capital, Brussels, is under its highest security alert -- "imminent threat" -- because of "quite precise information about the risk of an attack like the one that happened in Paris," the country's prime minister says.
Prime Minister Charles Michel made the announcement on November 21, referring to the terrorist attacks in Paris on November 13 that left 130 dead and hundreds injured. The Islamic State (IS) terrorist group has claimed responsibility for that attack.
Michel said security officials fear that "several individuals with arms and explosives could launch an attack...perhaps even in several places."
The city's metro and light-rail systems have been shut down and residents have been advised to remain indoors.
"We urge the public not to give in to panic," Michel said, adding that "we have taken the measures that are necessary."
Brussels police said they discovered a laboratory for the production of explosives during a raid in the capital on November 21. Weapons were also discovered at the site, but no explosives or suicide belts were found.
Police added that three people have been detained, but did not provide details.
Belgium's National Security Council held an emergency session on November 21. Extra security has been imposed on trains, in railway stations, and at airports.
Brussels, with a population of more than 1 million people, hosts the headquarters of the European Union and NATO, as well as many large corporations.
All soccer matches in the Brussels region have been canceled for at least November 21-22. Outside of Brussels, the rest of the country is on a Level 3 alert.
The suspected ringleader of the Paris attacks, Abdelhamid Abaaoud, was a Belgian of Moroccan origin. He was killed in a confrontation with French security forces in a Paris suburb on November 18.
Another Belgian citizen, suspected Paris gunman Salah Abdeslam, is still at large. Belgium has filed criminal charges against three of its citizens who were detained in Paris in connection with the attacks.
Officials around the world are investigating the mostly French and Belgian network that carried out the Paris attacks with help from Islamist radicals in Syria.
Meanwhile, police in Turkey have detained three suspected IS militants, including a Belgian citizen who is accused of acting as a scout for planners of the attacks in Paris.
Officials said Belgian Ahmet Dahmani, originally from Morocco, was detained on November 21 at a luxury hotel in Antalya. The other two men, both reportedly Syrians, were detained nearby.
The Syrians are suspected of being sent by IS to help Dahmani cross the border into Syria.
Turkish authorities earlier this week deported a group of eight Moroccans suspected of having ties to IS.
At the UN, the Security Council unanimously approved a French-sponsored resolution calling on countries to take “all necessary measures” against IS and redouble efforts to prevent further attacks by it and other extremist groups.
The resolution was passed by the 15-member council on November 20.
IS "constitutes a global and unprecedented threat to international peace and security,” the resolution said.
UN member states should "redouble and coordinate their efforts to prevent and suppress terrorist attacks" committed by Islamic, Al-Qaeda, and other terrorist groups, it said.
It also urges states to intensify efforts to stem the flow of foreigners looking to join with Islamic State in Iraq and Syria.