Passengers were seen fleeing from the National Express West Midlands vehicle as it burst into flames outside an Aldi on the Stratford Road in Sparkbrook, Birmingham.
Three fire engines rushed to the scene and two firefighters needed breathing apparatus to tackle the severe blaze which broke out at around 2pm.
Shocking pictures taken at the scene show the top deck of the number 6 bus completely ablaze with thick black smoke pouring out the windows.
Firefighters took just half-an-hour to put out the "severe" fire and confirmed it was being treated as a suspected arson attack and the road was re-opened by 3.30pm.
A spokesman for West Midlands Fire Service said: "We got the call at 2.10pm to the Stratford Road in Sparkbrook.
"It involved a severe fire on a double-decker bus. We had two firefighters with breathing apparatus and used two hose reel jets and a covering jet to stop the fire spreading.
"The stop message came through at 2.43pm and the incident is now believed to be arson so police are in attendance and will be dealing with it."
It came as firefighters in England and Wales took part in a 24-hour strike in a row with the government over pensions.
The walk-out, which started at 9am is the longest strike yet in the three-year dispute, and coincides with the start of the World Cup in Brazil.
The Fire Brigades Union (FBU) said the government's proposals were "unfair and unaffordable".
But the Department for Communities and Local Government accused the union of "disrupting" discussions.