The attack took place on Tuesday afternoon at Malmö’s central train station, prior to a derby match between Malmö FF and Helsingborg, the two main rivals from Sweden’s southern county of Skåne.
Police are classifying the crime as a serious assault.
“A so-called supporter has placed some sort of firecracker or rocket in her mouth and lit it. She received burns on her lips,” said Skåne police spokesperson Calle Persson to the TT news agency.
The incident was part of a volatile evening in which seven people were arrested for rioting and vandalism.
At least one person was arrested on suspicions of assaulting a police officer.
“He shot a firework at a police officer,” said Mikael Persson of the Skåne police.
A police dog was also injured after being hit by a thrown firework.
Before the match had started, a pub on Malmö’s Södergatan was vandalized by visiting fans from Helsingborg.
In addition, police stopped around 20 Danish citizens at the Öresund Bridge because of fears that they were heading to the match to instigate a brawl.
Ten of them were detained by police, while ten were returned to Denmark and three were released.
Following the match, which Helsingborg won 2 – 1, the disturbances continued in downtown Malmö. One man was reportedly assaulted near the Kronprinsen district, but he escaped without serious injuries.