The city's police department tweeted that the bus driver was rushed to the hospital with severe burns but no one else was known to have been injured.
The department said the incident was being investigated as a traffic accident.
Photos and video footage from the scene showed the bus in flames and thick black smoke.
Crazy scene here in Stockholm, Sweden! Still trying to find out all what’s happening but I am safe and on a tour bus enjoying this beautiful city! #stockholm #SpringBreak2K19 pic.twitter.com/uBX3IA9aKC— Brooke Davis (@MissDavisUS) March 10, 2019
Stockholm fire and rescue officials were called to the Klara Tunnel in central Stockholm at about 11:25 a.m. local time.
Rescue worker Asa Skold told Swedish newspaper Aftonbladet the bus appeared to have collided with barriers at the top of the shallow tunnel's entrance, causing a gas tank on the vehicle's roof to explode.
The newspaper said the out-of-service bus was on its way to a bus depot.