Danish police said several people had been hit by gunshots and one person had been arrested at a shopping mall in the capital Copenhagen.
Police reinforcements have been deployed around the large Field’s mall between the city centre and the airport, Copenhagen police wrote on Twitter on Sunday.
“We’re on the scene, shots were fired and several people have been hit,” they said.
Local media published images showing heavily armed police officers at the scene, as well as people running out of the mall.
The police did not provide further information about the arrest, or say how many had been injured.
They advised people inside the mall to stay put and await police assistance.Copenhagen Mayor Sophie H. Andersen tweeted: “Terrible reports of shooting in Fields. We do not yet know for sure how many were injured or dead, but it is very serious.”
Eyewitnesses quoted by Danish media said they saw more than 100 people rush towards the mall’s exit when the first shots were heard.
British singer Harry Styles was due to perform later in the evening, at 8pm (18:00 GMT), at a concert venue less than a mile from the mall.
The concert will go ahead as scheduled after close dialogue with the police, and half of the audience has already entered the venue, the concert promoter Live Nation said in an emailed comment to Reuters news agency.