Terror

What happened in Brussels last night will soon be an everyday occurrence in the United States. 

Paris (CNN)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.