The announcement by the Crisis Centre of the Belgian Interior Ministry is advising the public to avoid places where large groups gather -- such as concerts, sporting events, airports and train stations -- and comply with security checks. The rest of the nation will maintain its current terror level.
If people take the terror alert seriously, Brussels will be "shut down tomorrow," CNN terrorism analyst Paul Cruickshank said.
"It suggests they have something specific and credible at the intelligence front pointing them in the direction that there may be a terrorist plot in the works," he said. "It also suggests they don't have a handle on it, that they don't know where these plotters are or where they're coming from."
The increase in alert level for Brussels comes as authorities investigating last week's terror attacks in Paris conduct raids in Belgium as they work to identify and take down the network of terrorists behind the carnage.
Salah Abdeslam, 26, is the subject of an international search warrant. He was last seen driving toward the Belgian border when police stopped and questioned him a few hours after the attacks, not knowing that he was allegedly involved. His whereabouts are unknown.
ISIS has claimed responsibility for the Paris attacks.
Abdeslam is one of two brothers allegedly involved in last week's coordinated attacks at the Bataclan concert hall, outside the French national soccer stadium and at restaurants in Paris. Though he's a French national, he was born in Belgium.
That's one of several connections between this latest attack and Belgium, a country seen as fertile ground for jihadist recruiters. It's where members of a suspected terror cell waged a deadly gun battle in January with police and where three Americans in August overpowered a radical Islamist gunman on a Paris-bound train.
It was also home to suspected ringleader Abdelhamid Abaaoud. He was killed during a dramatic raid that shook the Saint-Denis neighborhood
outside Paris and collapsed an entire floor of an apartment building.
French Interior Minister Bernard Cazeneuve said Abaaoud "played a decisive role" in the Paris attacks and played a part in four of six terror attacks foiled since spring, with one alleged jihadist claiming Abaaoud had trained him personally.
He was once allegedly involved in gangs in Molenbeek. Belgian special operations forces raided that impoverished Brussels suburb, which has a history of links to terror plots, on Monday.
Thursday, Belgian authorities detained nine in raids around that country, the federal prosecutor's office said. Seven of those people were questioned after six raids around Brussels related to Bilal Hadfi, one of those who blew himself up outside the Stade de France.
State of emergency extended in France
The French Parliament on Friday completed its approval to extend the state of emergency for three months after terrorist attacks last week that left 130 people dead.
France's upper house, the Senate, unanimously passed the bill. The National Assembly, the lower chamber, overwhelming approved it Thursday. The next step is for the bill to be reviewed by France's constitutional council, which is not expected to cause any problems.
Authorities had been using the state of emergency declared by President Francois Hollande to carry out a widespread clampdown on potential terrorist threats, detaining dozens of people, putting more than 100 others under house arrest and seizing an alarming array of weapons.
In remarks to the Senate before the vote, Prime Minister Manuel Valls said 793 searches had been carried out across France in the past seven days. In all, 174 weapons, including "18 weapons of war," were seized, he said.
France has about 10,000 military personnel deployed across the country in addition to 100,000 police officers and gendarmes, plus 5,500 customs officials, Valls said.
Since the attacks last Friday, 164 people considered dangerous have been placed under house arrest, he said.
Meanwhile, two of those involved in last week's attacks entered Europe at the same entry point in Greece and on the same day, the prosecutor's office said. Both became suicide bombers outside entrances at Stade de France, one blowing himself at 9:20 p.m. and the other at 9:30 p.m.
U.S. national security officials told CNN that four of the Paris attackers were on the broad "watch list" of known or suspected terror suspects called TIDE (Terrorist Identity Datamart Environment), which has 1.1 million names. At least one of the suicide bombers was on the no-fly list before the attacks.
Valls announced that one more person has died following last week's attacks, bringing the death toll to 130.
Woman didn't kill herself
Meanwhile, the Paris prosecutor's office announced that Hasna Ait Boulahcen, the woman found dead after the police raid in Saint-Denis, did not blow herself up as preliminary information earlier suggested.
Rather, a man was wearing a suicide device that detonated, the prosecutor's office told CNN. Further details about him weren't immediately released.
Boulahcen, 26, was a relative of Abaaoud, official sources in France told CNN.
Friends of her family in their hometown of Aulnay-sous-Bois, on the northeastern outskirts of Paris, said she had lived there until recently. Residents in the area told CNN authorities had taken her mother and brother into custody. And the Paris prosecutor's office told CNN that police were searching the mother's home.
France wants wider anti-ISIS coalition
On Friday, the U.N. Security Council unanimously adopted a resolution penned by France that gathers international support for counterterrorism efforts, specifically aimed toward ISIS.
The resolution calls on all member states to take all necessary measures in compliance with international law to "redouble and coordinate their efforts to prevent and suppress terrorist acts committed specifically by ISIL" and urges states to "intensify their efforts to stem the flow of foreign terrorist fighters to Iraq and Syria."
Hollande said he would appeal to world leaders
to form a wider coalition to go after ISIS, including meeting next week with U.S. President Barack Obama and Russian President Vladimir Putin.
Russia has taken military action in Syria independently of the U.S.-led coalition, attacking ISIS but also other groups opposed to Syrian President Bashar al-Assad, a close ally of Moscow.
And France's interior minister, Cazeneuve, will press the case for more concerted European action, during an upcoming European Union justice council meeting in Brussels.
Talking to French lawmakers Thursday about extending the state of emergency, Valls warned that not taking steps to combat ISIS could have dire consequences. For proof, he pointed to their history of executions, bombings, beheadings and added that the use of chemical and biological weapons can't be ruled out.
"The way they are killing is constantly evolving," Valls said. "The grim imagination of those giving the orders has no limit."
Back to the bar
A week after terrorist attacks shattered a French Friday evening, Parisians grasped for the things they love -- cafes and bistros.
The search for normalcy in a city shaken to its core persisted even as suspects remained on the run and investigators desperately fought to keep terrorists from striking again.
The cafes and bistros of Paris are the French capital's living room.
At the coaxing of Hollande and restaurateurs, Parisians have rallied behind the hastag #TousAuBistrot, basically "everyone to the bistro" or "back to the bar."
"What would our country be without its cafes, concerts, sport events, museums?" Hollande asked this week.
Even as they try to get back to normal, Parisians maintain a wary eye on the latest developments.
Jean-Luc Perez had little choice but to shut his beauty salon in the wake of the attacks. It sits near La Belle Equipe bar, one of the terrorists' targets.
One of the attackers fired his Kalashnikov at the store while Perez was inside preparing to leave for the night. Three bullets shattered the plate-glass windows.
Nineteen people died and dozens more were wounded across the street. Despite those deaths, Perez was determined to reopen on Thursday.
"We want to beat the terrorists, and to do that we have to get back to life, back to normal," he told CNN. "We have to open the salon and get back to work."
Bennie Tucker, 21, originally from the United Kingdom, is working in the film industry in Paris. He lives near the scene of the attack on the Cafe Bonne Biere and the Casa Nostra pizzeria, an area where five people died in the attack, and was out drinking with a friend nearby when the gunmen struck.
A week later, he asserted Paris surely would recover.
"Paris is a huge city. It has to get back to normal, it can't just close. People are going to go back to work on Monday, and life is going to go on," he said.