UPDATE:  (NBC News) - A shooting at a popular shopping center Friday evening in Munich, Germany, killed at least six people, injured several others, and brought the city to a standstill as police hunted for three attackers with long guns.

"This is a terror situation," Munich police spokesman Marcus Da Gloria Martin said in a press conference.

Police confirmed to NBC News that they were searching across the city for three suspects.

"At the moment, we don't know where the perpetrators are. Take care and avoid public places," Munich police tweeted.

According to a police spokesperson, the shooting started at a McDonald's attached to the Olympia Einkaufszentrum shopping center before the shooters made their way into the mall. The spokesperson said it wasn't clear whether the suspects were still in or near the mall or if they had gone elsewhere.

Tweeting in German for locals and English and French for tourists, Munich police pleaded, "Please avoid public areas."

All Munich public transportation — subway, bus, and tram — was shut down after the shooting, according to Munich Transport Corporation MVG.

A major police operation, which included German special forces, was underway at the mall.

Munich police said there were "unconfirmed reports" of more violence elsewhere in Munich, but said the "situation is unclear."

Thamina Stoll, a Duke University student from Munich, was heading to the shopping center with her family when she saw people flee.

"People were running and telling us not to go to the shopping mall. They were in shock. They weren't able to talk anymore," Stoll told NBC News.

The sprawling two-story Olympia Einkaufszentrum opened in 1972 during the Summer Olympics in Munich, according to the mall's website.

The shooting is the second attack in Germany's Bavaria region this week. On Monday, a 17-year-old Afghan injured four people with an ax on a German train near Wuerzburg-Heidingsfeld before he was shot dead.

It wasn't immediately clear whether the mall shooting was tied to terrorism, but it put other cities on alert.

The New York Police Department's counterterrorism unit tweeted that it was "monitoring events unfolding in Munich." And the United Kingdom's Foreign Ministry urged Britons to "avoid this location and follow the advice of local authorities" on its travel advisories page.

The U.S. Consulate in Munich tweeted, "Closely follow police instructions and shelter in place."