Central Brussels was almost empty on Saturday night as a terror alert led restaurants and bars to shut early amid fears of a Paris-style attack.
Soldiers patrolled the streets as a manhunt continued for the fugitive Salah Abdeslam, who was thought to be armed with a suicide belt.
Reports said he was in the Brussels area and trying to get to Syria.
The city was a base for the attackers - Islamic State militants - who killed 130 people in Paris.
Belgian Prime Minister Charles Michel said there was "quite precise information" that "several individuals with arms and explosives could launch an attack... perhaps even in several places".Abdeslam appeared to have a large jacket and may have been ready to blow himself up, one of the men who drove him to Belgium has told his lawyer.
Carine Couquelet told Belgian TV this raised questions, including the possibility that Salah Abdeslam may have been supposed to blow himself up in Paris but had had second thoughts.
Friends of Abdeslam told ABC News they had spoken to him on Skype and said he was hiding in Brussels and desperately trying to get to Syria.
They said he was caught between European authorities hunting him and so-called Islamic State members who were "watching him" and were unhappy that he had not detonated his suicide belt.
The Belgian authorities responded by placing Brussels on its highest level of alert.
Residents were told to avoid crowds, the metro was closed along with cinemas and shopping malls, cafes and restaurants were asked to shut at 6pm and soldiers deployed on the streets.
The US embassy told Americans in the country to stay indoors and the US European Command issued a 72-hour restriction on travel to the city by all military personnel and contractors.The government would review the security situation in Brussels on Sunday afternoon, Michel said.
Meanwhile on Saturday, members of the US rock band Eagles of Death Metal described the horror of the Bataclan concert hall massacre in their first interview since the attack.
Lead singer Jesse Hughes said that a group of fans who hid in the band's dressing room were found by the gunmen and slaughtered, all except for one who hid under Hughes's leather jacket.
In a clip from the interview with Vice News, which will be released in full next week, an emotional Hughes said: "A great reason why so many were killed was because so many people wouldn't leave their friends. So many people put themselves in front of people."
In Turkey, a Belgian man of Moroccan descent, Ahmad Dahmani, 26, was arrested at a luxury hotel in Antalya along with two other terror suspects, Turkish authorities told the BBC.
Dahmani is believed to have been in contact with the suspects who perpetrated the Paris attacks, an official said.
He arrived from Amsterdam on 14 November; there is no record of the Belgian authorities having warned Turkey about him, which is why there was no entry ban, the official said.
The Belgian authorities have so far charged three people with involvement in the Paris attacks, claimed by Islamic State militants.
French media have reported that nine militants carried out the attacks, and seven died on Friday night. So it is possible that another attacker - as well as Salah Abdeslam - is still at large.
Earlier, the UN Security Council adopted a resolution to "redouble" action against IS following last week's deadly attacks in the French capital.
What is Islamic State?
IS is a notoriously violent Islamist group which controls large parts of Syria and Iraq. It has declared its territory a caliphate - a state governed in accordance with Islamic law - under its leader Abu Bakr al-Baghdadi.
What does it want?
IS demands allegiance from all Muslims, rejects national borders and seeks to expand its territory. It follows its own extreme version of Sunni Islam and regards non-believers as deserving of death.
How strong is IS?
IS projects a powerful image, partly through propaganda and sheer brutality, and is the world's richest insurgent group. It has about 30,000 fighters but is facing daily bombing by a US-led multi-national coalition, which has vowed to destroy it.