There were no passengers on the bus at the time of the incident, which is thought to have been caused by a gas tank explosion.
The driver was taken to hospital but Stockholm Police tweeted
that the extent of the driver's injuries was “unclear”.
Anders Palm, a spokesman for the Greater Stockholm Fire Brigade, told news agency TT that the bus was completely engulfed in flames and the fire burned itself out. Investigators believe that a biogas tube mounted on top of the bus may have caused the powerful fire.
“Our guess is that the bus had gas tubes on its roof that ran into something. It is a gas bus, so that is one of our initial theories. This happened just after the bus drove under a tunnel,” Palm said.
The accident happened as the bus was entering Klaratunneln, a tunnel at the Tegelbacken traffic junction in central Stockholm. TT reported that a similar accident occurred in May 2013, but in that case the biogas tubes on top of the bus fell off but did not explode. Like the previous incident, it is believed that Sunday's bus fire was caused by the vehicle's roof-mounted gas tubes colliding with a height warning sign.
Police officers wearing gas masks were on the scene to keep people out of the area and Centralbron was completely shut down to all traffic.
Thick smoke emanated from the accident site and could be seen over large parts of central Stockholm.
Smoke from the bus fire could be seen over much of Stockholm on Sunday. Photo: Susanne Kivinen/TT