A motorist brought a runaway bus to a halt as it careered towards a group of children by ramming the 10-tonne vehicle with his own car.
Dean Sutcliffe, 33, from Thetford, Norfolk, drove his Vauxhall Zafira into the path of the empty bus as it rolled along Bank Plain in Norwich.
He managed to stop it before clambering aboard and jamming on the handbrake.
Norfolk Police are investigating claims two youths were seen tampering with the bus before it started moving.
Mr Sutcliffe said he had seen the bus set off and it was bouncing off lampposts and bins towards a crossing.
He explained: "I braked my car and stopped it in the crossing.
"The bus came down and caught the front corner of my car and that's when I noticed there wasn't an actual bus driver on the bus itself.
"I used my car to nudge the bus around the corner and brought it to a stop and put the handbrake on with the help of a pedestrian who I later found out to be an off-duty policeman."
Roger Haywood, from First Bus, said Mr Sutcliffe had been "bold and brave".
Mr Haywood said the bus was waiting for a driver changeover with its engine turned off, but the second driver was late.
"Our concern is that youngsters or even mothers with prams could have been injured," he said.
"If some youths or youngsters did get onto the bus to start it from its standing position on Castle Meadow and then steered it onto Bank Plain before jumping off, then this was a reckless and criminal action."
Mr Haywood also promised Mr Sutcliffe would be compensated for the damage to his £12,000 car.