The boys are responsible for at least eight cases of sexual assault, according to a report in the local TTELA daily.
In several cases the assaults have targeted young girls.
"The boys have got together in a group of around five people and selected a victim whom they have surrounded," said Sten Melander at Trollhättan police to the newspaper.
The matter came to the attention of the police shortly before the Midsummer holiday following reports that a young girl and boy had been assaulted.
The police suspected that a spate of recent similar attacks that had occurred in the town since April may be connected.
Sten Melander told the newspaper that the boys' young age made the case exceptional.
"They are eight to 12-years-old, in various constellations. It is very unusual that such young children build gangs," he said.
The attacks have occurred in public places and the majority of the victims are girls aged 10-16-years-old. Some adults have also been targeted for verbal abuse and been spat at.
The police have now handed over the matter to the social services.
"They are so young, so there will be no criminal proceedings. The social services have the case, we are merely providing the investigation," Sten Melander said to TTELA.
"The social services expect the parents to deal with this," he added.
The age of criminal responsibility in Sweden is 15-years-old.