There was no passenger in the vehicle at the time of the mishap, he said.
As the bus was moving from Mumbra to Anand Nagar in the city, its lights suddenly went off near Retibunder around 8.30 pm yesterday, Thane's regional disaster management cell chief Santosh Kadam said.
The driver stopped the vehicle and he along with the conductor got down to check what was the problem.
However, suddenly the bus caught fire with a loud sound and got charred within minutes, he said.
An investigation was on into the mishap, he added.