Three fire crews were sent to tackle a bus on fire outside the Royal Preston Hospital.
Sharoe Green Lane was temporarily shut while a blaze in the engine compartment of the single-decker was extinguished.
It is thought that all passengers were safely evacuated from the vehicle when smoke was noticed just after 3.30pm.
The fire is thought to have started because of a mechanical malfunction.
Preston Bus confirmed delays to services 19A, 23, 88A, 88C were “likely for some time” and apologised for any inconvenience.
Police have contacted Lancashire County Council to clean a fuel spillage on the road.