Brussels on security clampdown; new arrests in Paris probe

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.

At 10 p.m. in Brussels (4 p.m. ET) most bars were closed or were in the process of closing.

Brussels' subway system will be closed until at least Sunday afternoon, when the threat will be reevaluated, Michel said.

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.

But the man arrested was later released, said Eric Van Der Sypt, spokesman for the federal prosecutor. He also said the raid produced only paintball guns and no explosives.

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,