Belgian authorities closed down Brussels' subway system and flooded the streets with armed police and soldiers Saturday in response to what they said was a threat of Paris-style attacks.

The decision to raise the threat alert to the highest level in the Belgian capital came as the manhunt continued for a suspect missing since the carnage in neighboring France. It was taken "based on quite precise information about the risk of an attack like the one that happened in Paris," said Belgium's Prime Minister Charles Michel.

The tip authorities received suggested that an attack would involve "several individuals with arms and explosives launch actions, perhaps even in several places at the same time," he said.

The U.S. Embassy in Belgium urged Americans in the country "to shelter in place and remain at home" while the U.S. European Command issued a 72-hour travel restriction for U.S. military personnel on travel to Brussels -- a city of more than 1 million that is home to the headquarters of the European Union, the NATO alliance and offices of many multinational corporations.

On Saturday night, a relative calm descended on the city center, where restaurants and beer bars would usually be teeming with business. On Brussels' central square, the Grand Place, tourists snapped selfies as a green army truck full of soldiers pulled up next to a lit Christmas tree. Some restaurants and bars shuttered their doors, while others remained open, defying advice from the mayor to close for the night.

Tensions were also high elsewhere in Europe. In Paris, police equipped with emergency powers extended a ban on demonstrations and other gatherings through Nov. 30, when a U.N. climate conference with more than 100 heads of state is scheduled to start.

Several of the Paris attackers had lived in Brussels, including suspected mastermind Abdelhamid Abaaoud, who was killed in a standoff with French police on Wednesday.

Salah Abdeslam, another suspected attacker, is at large and is known to have crossed into Belgium the morning after the Nov. 13 attacks. A Paris police official and the Paris prosecutor's office said Saturday they had no firm information on Abdeslam's whereabouts, including whether he was in the Brussels area.

Carine Couquelet, the lawyer for a man who rode back to Belgium with Abdeslam, said in a broadcast interview Saturday that her client had said Abdeslam was extremely nervous and may have been wearing a suicide bomb vest.

The Belgian Federal Prosecutor's office said several weapons were discovered during a search of the home of one of three people arrested in connection with the Paris attacks, but that no explosives were found. It didn't specify when the weapons were found. The three have been charged with "participation in terrorist attacks and participation in the activities of a terrorist organization."

In the Turkish coastal city of Antalya, authorities detained a 26-year-old Belgian citizen, suspected of being connected to Islamic extremist and possibly to the Paris attacks -- raising hopes of a possible new break in the investigation.

The private Dogan news agency identified him as Ahmet Dahmani and said he is suspected of having explored areas in Paris that were targeted in the attacks. A senior Turkish government official, who spoke on condition of anonymity due to Turkish government rules, said he was believed to have been in contact with the Paris attackers. Dahmani had arrived in Turkey Nov. 14 from Amsterdam, and was preparing to cross into Syria together with two other suspected Islamic State militants, the official said.

Officials across Europe, the Mideast and in Washington are trying to determine how a network of primarily French and Belgian attackers with links to Islamic extremists in Syria plotted and carried out the deadliest violence in France in decades -- and how many may still be on the run.

Belgium's national Crisis Center raised its terrorism alert for the Brussels region to Level 4, which indicates a "serious and immediate threat." Heavily armed police and soldiers on Saturday patrolled key intersections of the city.

Residents were advised to avoid gatherings, train stations, airports and commercial districts. Service was halted on the Brussels Metro, as well as on streetcar lines that run underground.

As the first snow flurries of the season fell, many stores and commercial centers in the capital shut their doors on what normally would have been a busy weekend shopping day in the lead-up to the Christmas and New Year's holidays. The Royal Museums of Fine Arts of Belgium closed for the weekend, and a Saturday evening concert by French rocker Johnny Hallyday was rescheduled for next March, the Palais 21 venue announced on its website.

Brussels Airport, which is not in the Brussels administrative region, reported normal operations Saturday, but external communications manager Florence Muls said special attention was being paid to security.

The prime minister, speaking at a news conference after the emergency government meeting, said "We urge the public not to give in to panic, to stay calm. We have taken the measures that are necessary."

Michel said the government's crisis cell would meet again on Sunday afternoon to reassess the threat.

William Bridell, an American living in Brussels, said there was a noticeable difference in the city Saturday.

"There's definitely not as many tourists and the Belgians have stayed in," he said. He doubted there would be any attacks with the greater police presence and was on his way to watch a football match.

British tourists Georgie Maughan and Alexander Walker said they weren't too worried about the security situation and had dismissed advice from their hotel to avoid crowded places. They, too, were in search of an open sports bar to watch a football game.

While some restaurant staffers weren't happy about having to shut down on one of the busiest nights of the week, waitress Lourdes Taipe said it was probably for the best.

"It's very bad for the company because we're losing money but we have to do it for the security of the customers," she said, adding the presence of so many police and military officers in central Brussels made staffers feel reassured.

Parisians were still trying to come to terms with the new reality imposed on them by the attacks, including security checks on entering stores and public buildings and a visible presence of armed police around the city.

In one of many tributes paid to the 130 victims of the Paris attacks over the past eight days, members of the Federation of Bikers of France gathered in Paris for a "RIP run." Some 75 bikers from Paris and surrounding areas drove through the city and held a minute of silence in front of the Bataclan concert hall where 89 people were killed, including three members of their Federation.

The annual Christmas market along the Champs-Elysees avenue, shuttered after the attacks, reopened in time for the weekend. Although crowds seemed smaller than usual, people lined up on the cobblestones to ride a huge ferris wheel at the Place de la Concorde, lit up in red white and blue.