Two yellow and grey trains stood at the site, both with visible damage to the front and facing each other on a tree-lined stretch of track, with rescue services describing "chaotic" scenes inside the carriages.
"The two trains collided head-on, causing large damage to them and sending broken glass flying everywhere," fire and rescue service leader Christoffer Buhl Martekilde told reporters on Thursday.
A police spokesperson said it was too early to say what had caused the crash, on the Gribskov train line linking the towns of Hillerod and Kagerup, adding there would be an extensive investigation.
A total of 37 people are believed to have been on board the two trains at the time of the crash, which occurred shortly before 6.30am, police said.
While health services attended to the injured, those who were not physically hurt were also offered assistance, said transport group Movia, parent company of rail operator Lokaltog.
"The serious train accident on the Gribskov Line deeply affects us at Movia, and our thoughts go out to the injured, their relatives and Lokaltog's employees," Movia board chair Kenneth Gotterup said in a statement.
Emergency services scrambled 18 vehicles and 47 rescue workers to the crash site.
Swedish Prime Minister Ulf Kristersson said he had contacted acting Danish Prime Minister Mette Frederiksen to offer assistance.
Danish police said they had declined the Swedish offer to assist as local resources were sufficient.
"There are injuries among the passengers. Everyone is out of the trains, so no one is trapped ... Large resources have been dispatched to the scene," a spokesperson told Reuters earlier.
Denmark's worst railway disaster this century occurred in 2019, when a high-speed train hit an obstacle on a bridge during a storm, killing eight people and injuring many more in what police said was a difficult rescue operation.
While such accidents are rare in Denmark, one person was killed and several others were injured in 2025 when a train hit a vehicle and derailed in the south of the country.
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