STOCKHOLM - A major police operation was underway in northern Sweden on Wednesday after knife-wielding men threatened passengers aboard a train.

The incident took place late Tuesday when two men threatened passengers on a night train between the northern towns of Lulea and Umea. One of the men apparently had a knife.

A witness told local newspaper that when the train arrived at Umea's Ostra train station, heavily armed police officers had blocked both entrances to the station.

Another witness said passengers on the train had to run as police warned they were at risk.

"It felt serious," one witness told the newspaper Aftonbladet. "The police shouted at people walking towards the train to turn around. They shouted that it was dangerous and that everyone should leave the area."

Two men, both in their thirties, were arrested. Local police took the men for questioning and eventually released both of them.