Brussels will remain on the highest alert level and maintain security measures that have severely disrupted normal life in the city until at least Monday as it faces an "imminent" threat of attack, Belgium's prime minister said Monday, urging his population to remain calm.

A French police official says that an explosive vest found in a suburb on the southern edge of Paris contained bolts and the same type of explosive used in explosive vests in the Paris attacks.

The official said the vest — without a detonator — was found Monday by a street cleaner in a pile of rubble in Chatillon-Montrouge, just south of the capital.

The official said laboratory analysis showed that the explosive material was TATP — used in seven other explosive vests in the attacks that killed 130 people and wounded hundreds of others. The official wasn't authorized to speak publicly and could not be identified by name.

In addition, the vest was found in the same zone where the cellphone of a man sought by police was geolocalized. Police have been conducting a manhunt to find Salah Abdeslam, but it was not known if the explosive vest was abandoned by him. He was stopped by police in northern France after the attack but allowed to continue his journey to Belgium.

Belgian Prime Minister Charles Michel said the country's capital, which also houses the headquarters of the European Union and NATO, faced a "serious and imminent" threat of attack, while the rest of the nation would stay at the second-highest level. The country's crisis centre said the alert level would only change this week if a significant breakthrough by investigators warranted it.

The increased security measures in the wake of the Nov. 13 Paris attacks that killed 130 people and wounded hundreds have virtually shut down the city, with the subway and shops closed over the weekend and schools remaining shut on Monday. Michel said that despite the alert level, schools and the subway system would reopen progressively as of Wednesday.

Fighters launch from Charles de Gaulle aircraft carrier for strikes against Islamic State targets in Iraq 0:40

"We are very alert and call for caution," Michel said. "The potential targets remain the same; shopping centres and shopping streets and public transport."

"We want to return to a normal way of life as quickly as possible."

The unprecedented security measures come as authorities hunt for one or more suspected extremists, including Salah Abdeslam, a fugitive since being named as a suspect in the Paris attacks. He has yet to be found.

Belgian authorities arrested a suspect on terrorism related charges Monday after a series of raids that saw 21 people detained between Sunday night and midday Monday.

The federal prosecutor said the suspect, who was not identified, was charged with terrorism offences related to the Paris attacks after authorities detained 16 people Sunday. Three others were charged with similar offences last week. The other 15 people detained Sunday evening were released.

'Otherworldly feeling' in Brussels

Brussels is more accustomed to the toing and froing of European diplomats than to truckloads of soldiers patrolling the streets. Its stores and markets should be bustling in the runup to Christmas. But the terror alert has created a very different atmosphere.

"It's fitting since Belgium is the birthplace of surrealism but there is an otherworldly feeling here," said Jan Van Gent, a retired pharmaceutical manager. "This is like a Magritte painting, to see these soldiers in a nice town where they don't belong."

Others weren't convinced the government's actions were anything more than a PR move.

"They have a lot of attention on them now so they have to prove they're doing something, but I don't know how much difference this will make," said Maxime Legena, an IT technician. "We don't really know how big this threat is because the government hasn't said very much."

The Belgian capital Brussels has entered its third day of lockdown, with schools and underground transport shut and more than 1,000 security personnel deployed across the country. (Michael Probst/Associated Press)

But he did appreciate one unintended side-effect of the attempts to shutter the city: "My drive to work was much faster today because there's nobody on the streets."

Brussels native Jacqueline Vander-Poelen, who lives near the city's Medieval Grand Place square, said the noise of police operations and reporters scurrying to cover them made for an annoying weekend.

"What's making me scared is the alarmist journalists saying there's going to be another attack," Vander-Poelen complained. While she couldn't remember any other instance where the city shut down so completely, she said residents would likely accept whatever measures were necessary to defeat extremists.

Some Belgians have developed their own style of acceptance, however. When Belgian authorities asked people to stop commenting on ongoing police raids on Sunday, social media users responded by flooding Twitter with pictures of cats, including some snaps of kitties holding up their paws like captured suspects.

Nishith Mehta, a tourist from Mumbai, was taking sunrise photos of the Christmas tree inside Grand Place and said the military presence was reassuring.

"It is better to be cautious than [to] see what happened in Paris," she said.

While high school student Michel Evangeline appreciated the day off and was busy taking photos of soldiers downtown, he was convinced his terror-prompted holiday wouldn't last long.

"They cannot shut down everything like this," he said. "Even the McDonald's is closed, and that is very strange."

A Foreign Affairs Department spokesperson told CBC News that due to the high threat level in Brussels, the Canadian Embassy will be open only for emergency consular services. The department has updated its travel alerts to indicate a high threat level in Brussels.

France's Defence Ministry said it had launched its first airstrikes from the Charles de Gaulle aircraft carrier, bombing IS targets in the Iraqi cities of Ramadi and Mosul in a seven-hour operation. The ministry said that four Rafale fighter jets were sent from the carrier on Monday afternoon, with two each flying over each city. France has already carried out strikes against IS targets in Syria.

Raids began Sunday

The raids in Belgium began late Sunday, capping a tense weekend that saw hundreds of troops patrolling streets.

Frank Foley, lecturer in war studies at King's College London and a terrorism expert, said it was difficult to know if what Belgium was doing was justified because authorities have provided so few details about their decision.

The measures intended to avoid attacks similar to the ones that caused devastating carnage in Paris could even be counterproductive if they last too long, he said.

"If these dramatic measures continue in Brussels, we will be doing the terrorists' job for them," Foley said. "The government may be unintentionally contributing to the atmosphere of fear."

But Henry Willis, director of RAND Homeland Security and Defence Center, likened the clampdown to the reaction of U.S. authorities after the Boston marathon bombing.

"They did shut down the city for a couple of days and when they lifted those restrictions, that's when they caught the terrorist," he said.

Several of the Paris attackers had lived in Brussels, including Abdelhamid Abaaoud, the man who authorities say orchestrated the plot. He was killed Wednesday in a standoff with French police.

French police issued a new public appeal for help in identifying the third attacker who was killed in the assault at the national stadium. They posted a photo of the man on Twitter Sunday asking the public for information.