A gunman opened fire inside a busy shopping mall in the Danish capital on Sunday, killing three people and seriously injuring three others, police said.

Copenhagen police inspector Soren Thomassen told reporters a 22-year-old Danish man had been arrested after the shooting, adding that there was no indication that anyone else was involved in the attack, although police are still investigating.

Thomassen said it was too early to guess the motive for the shooting, which took place last afternoon in Field's, one of Scandinavia's largest shopping malls and on the outskirts of the Danish capital.

The shooting comes a week after a mass shooting in neighboring Norway, where police say a Norwegian man of Iranian descent opened fire during the LGBTQ festival, killing two and injuring more than 20.

It was the worst gun attack in Denmark since February 2015, when a 22-year-old man was killed in a gunfight with police after a shooting in the capital that left two dead and five police officers injured.