Six people have died in a collision between two buses on the main road 288 near Rasbo, north of Uppsala.
A further 9 people are have been seriously injured, and 42 people have minor injuries.
The accident happened on a straight stretch of road with forest on both sides.
The buses were both running on route 811. One bus was travelling from Öregrund to Uppsala; the other bus was travelling in the opposite direction.
The buses came off the road and were left standing on a slope by the side of the southbound lane, according to news agency TT. The windscreen and side of one of the buses was entirely torn off.
Most of the injured were taken to Uppsala University Hospital, while two of those most seriously injured were taken by helicopter to Söder Hospital in Stockholm.
Those killed were mostly young adults, according to Uppsala University Hospital. No children are among the dead.
One of the bus drivers was taken to hospital in Uppsala for treatment. The other driver sustained only minor injuries and was being looked after by colleagues.
Both drivers worked for bus operator KR-Trafik.
“This is a terrible tragedy,” said the company’s CEO Gunnar Sahlén.
“We in the company management are set to head down from Östersund to Uppsala,” he added.
Uppsala police still do not know what caused the crash. Members of the Swedish Accident Investigation Commission were expected to visit the site at Tuesday lunchtime. The bus drivers have not yet been interviewed about the crash.
According to a study by the Swedish National Roads Administration, the road where the accident happened was of poor quality for the speed limit.
The part of the road that is in the worst state has a speed limit of 70km/h, while the rest of the road has a limit of 90km/h, despite the fact that the road is not suitable for vehicles travelling at that speed, the administration wrote in the study.
It is snowing at the scene of the crash, with temperatures around 0 degrees centigrade.
All survivors had been transported from the scene by about 9:15am. Forensic experts arrived on the scene shortly after 9am in order to approve the removal of the bodies to mortuaries.
Emergency services launched a major operation after being alerted to the crash.
Key members of the local authority gathered at a crisis centre in the area and emergency personnel were called in from bordering counties.
According to Aftonbladet, the buses were both running on route 811. One bus was travelling from Öregrund to Uppsala; the other bus was travelling in the opposite direction.
Concerned relatives can call Uppsala Police in 018 124 623.