Multiple people were wounded in firing "incidents" in the heart of the Scanian town, which is home to some 30,000-40,000 inhabitants, British media outlet Mirror UK said.
"We have received several reports of a shooting in Trelleborg", a police spokesperson was quoted saying by United Kingdom media.
The spokesman said: "A major police operation is underway".
Trelleborg, known for its past gang violence, is located 33 kilometres south of Malmo and 64kms southeast of Copenhagen, Denmark.
"We were quickly on the spot and found that, within a fairly limited area, we find more people with suspected shot injury". But we are at the scene and investigating'.
Trelleborg has been placed on lockdown by armed cops with reports that at least four people suffered gunshot wounds.
One witness told The Express: "There were many shots".
Local police are urging residents to stay away from the area. "We saw someone running outside", an eyewitness told the Swedish publication Expressen. A forensics team and canine unit have been sent to the scene.