One of the boys was taken to hospital in an ambulance, while the other made his own way to a nearby doctor’s surgery.
A number of passers-by witnessed the incident, one of whom alerted the police.
“According to the report, two 18-year-old men were the victims of a knife attack at the hands of six to eight perpetrators aged 16 to 18. An object that appeared to be a firearm was also seen. We got a very good description,” said police spokeswoman Ann-Charlotte Wejnäs.
Police in the Norrmalm district have sent a team of six investigators to the scene.