PARIS -- Belgium's capital was under the country's highest terror alert level Saturday -- with Brussels' subway service suspended and people warned to avoid gatherings -- as authorities warned of a possible imminent threat a week after deadly attacks in Paris.
Specific reasons for Friday night's alert weren't released, but as news of the heightened security measures spread, so did information about four new arrests tied to the Paris probe -- one in Brussels' Molenbeek suburb, and three in Turkey.
The Belgian Interior Ministry's crisis center cited only "a serious and imminent threat" when it announced Brussels' terror alert level was rising to 4, the country's highest. But Prime Minister Charles Michel said authorities had reason to suspect possible attacks in more than one location.
"We are talking of a threat of several individuals with weapons and explosives, to launch acts, maybe even in several places at once," Michel told reporters on Saturday.
Brussels' subway system will be closed until at least Sunday afternoon, when the threat will be reevaluated, Michel said.
At 10 p.m. in Brussels (4 p.m. ET) most bars were closed or were in the process of closing.
The government advised the public to avoid places in the capital where large groups gather -- such as concerts, sporting events, airports and train stations -- and comply with security checks. Michel said authorities' main objective is to reduce the number of large events to free up police officers to secure Brussels.
Brussels' streets, while not empty, were relatively sparse. Armed security officers wearing camouflage could be seen on the streets and in front of metro stations.
Outside of Brussels, the nation will maintain its current terrorism level.
Arrests in Belgium, Turkey
Meanwhile, news spread of Friday arrests in Belgium and Turkey that authorities said were linked to investigations into last week's attacks that killed 130 people in the Paris area.
In the Brussels suburb of Molenbeek, police arrested one person and searched a home, Belgium's federal prosecutor's office said. A few weapons were found, but no explosives, the office said. No further information about the arrest or the suspect was released.
Authorities say Molenbeek, an area with a history of links to terror plots, was a home base to some of the Paris attackers, including two brothers. Those brothers were Ibrahim Abdeslam, who died in the attacks, and Salah Abdeslam, who police say is on the loose.
Turkish authorities arrested three people with suspected ties to ISIS, including a Belgian man who Turkish investigators believe was in contact with the Paris attackers, a Turkish official said.
Ahmet Dahmani, 26, a Belgian national of Moroccan descent, was arrested at a hotel in Antalya, CNN Turk reported. Two other suspects, Syrian nationals Ahmet Tahir, 29, and Mohammed Verd, 23, were arrested after they traveled from Syria to meet Dahmani, authorities said.
The two were going to transport him to Syria, authorities said.
Dahmani arrived in Turkey from Amsterdam the day after the Paris attacks, the Turkish official told CNN on Saturday on condition of anonymity.
The official said Dahmani was able to enter Turkey because no country had notified Turkey to watch for him. The arrest was made based on intelligence that Turkey had gathered, the official said.
"Had the Belgian authorities alerted us in due time, Dahmani could have been apprehended at the airport," the official said. "We urge our allies to continue sharing information with us."
In another development, French investigators have isolated the DNA of the suicide bomber who died during Wednesday's intense police raid in Saint-Denis, outside Paris, a police source said, according to CNN affiliate BFMTV. The source said the bomber is not known by French police, French media said.
Brussels alert: Why now?
The increase in the alert level for Brussels comes after authorities conducted a number of raids in the capital and across the country after the attacks in France, Belgium's southern neighbor. They are working to identify and take down the network of terrorists behind the carnage.
Among the effects: One of four Belgian top-division soccer games scheduled for Saturday was canceled.
The match between Sporting Lokeren and Brussels-area club Anderlecht was to take place in Lokeren, about 50 miles northwest of Brussels. The Belgian Pro League released a statement saying the match was canceled in part because Brussels police officers who'd been scheduled to travel to the game to provide security had to remain in the capital because of the alert there.
The alert suggests authorities "have something specific and credible at the intelligence front pointing them in the direction that there may be a terrorist plot in the works," CNN terrorism analyst Paul Cruickshank 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," he added.
The U.S. State Department advised Americans to be cautious.
Salah Abdeslam on the run
Salah Abdeslam, 26, is the subject of an international search warrant. He was last seen on the night of the November 13 Paris attacks, driving toward the Belgian border. Police stopped and questioned him a few hours after the attacks, but let him go, 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. Although he is a French national, he was born in Belgium.
That is one of several connections between the Paris attacks and Belgium, a country seen as fertile ground for jihadist recruiters. Members of a suspected terrorist cell waged a deadly gun battle in January with police in Belgium.
The country was home to Abdelhamid Abaaoud, who is suspected of having been the ringleader of the Paris attacks. Abaaoud was killed Wednesday during the police raid in Saint-Denis.
French Interior Minister Bernard Cazeneuve said Abaaoud "played a decisive role" in the Paris attacks and played a part in four of six terrorist attacks foiled since spring, with one alleged jihadist claiming Abaaoud had trained him personally.
Abaaoud was once allegedly involved in gangs in Molenbeek. Because of Molenbeek's links to terrorist plots, Belgian special operations forces conducted raids there Monday.
On Thursday, Belgian authorities detained nine people in raids across the country, the federal prosecutor's office said. Seven of them were questioned after six raids around Brussels related to Bilal Hadfi, one of the November 13 Paris attackers. Hadfi, authorities say, blew himself up outside the Stade de France just outside Paris during a France-Germany soccer game.