At least four people were shot dead on a train in the US city of Chicago on Monday, police said, with a suspect apprehended soon after the mass shooting occurred.

Police said they responded to a 911 emergency hotline call early on Monday morning, arriving at the Forest Park train station where they found the four victims.

"Three were pronounced dead on scene, the fourth was transported to Loyola University Medical Center in Maywood, where they were later pronounced," a police statement said.

Gun violence is common in the United States, a country where there are more firearms than people. Attempts to clamp down on gun rights are always met with stiff political resistance.

Chicago police put out a description of the suspected offender based on surveillence footage, and a suspect was later taken into custody with a firearm also recovered, the statement said.

"This appears to be an isolated incident with no immediate threat to the community," the statement said.

Chicago operates one of the largest public transportation networks in the United States, with more than 317,000 people using its train system on an average weekday, according to the Chicago Transit Authority.

"It's a horrible situation. It's definitely something that you don't want to wake up to. It's a Monday morning on a holiday," police official Christopher Chin told reporters, referring to the Labor Day holiday being observed in the United States.

This year, there have been at least 378 mass shootings -- defined as a shooting involving at least four victims -- across the United States, according to the Gun Violence Archive.

At least 11,463 people have been killed in firearms violence, according to the GVA.

In June, the US surgeon general issued a landmark advisory declaring gun violence a "public health crisis" and calling for wide-ranging firearm controls that have historically been quashed by political opposition.

Firearms in recent years have become the leading cause of deaths for Americans aged between one and 19, above motor vehicles, the report said.