The attack took place shortly after 8pm behind the National Library (Kungliga biblioteket) in Humlegården park in central Stockholm.
The girl was found injured by a passerby who then notified police.
“She was in shock but we’ve got a description of the two men,” duty officer Ulf Bajas of the Norrmalm district police told the TT news agency.
Despite obtaining a description from the victim, police have nevertheless turned to the public in the hunt for more leads.
“If you’ve seen or heard anything, get in touch as soon as possible,” police spokesperson Jan-Olov Lundgren told the Expressen newspaper.
Police suspect two men were involved in the attack, which took place between 8pm and 8.30pm on Monday evening.
“We don’t know exactly how the rape took place. It appears she was attacked and thrown to the ground. Physical violence was used, but we don’t know if they had some sort of object or weapon,” police spokesperson Anna Johansson told the newspaper.
Following the attack, the girl was taken to hospital for treatment and to have her injuries documented.
A forensic investigation of the crime scene is underway and is expected to continue throughout the day on Tuesday.