The disorder happened during Sunday’s Kill the Bill demonstration in Bristol.
Avon and Somerset Police said the driver moved the van to shield colleagues who were “being attacked”.
He described smelling burning and hearing reports people were trying to set the tyres alight as the violence unfolded.
The footage shows the moment a lit item was placed under the vehicle.
An image of the man officers want to question has also been released.
Eight people have so far been arrested on suspicion of violent disorder following the clashes in the city centre. One man has been charged with possession of an offensive weapon.
Those held on suspicion of violent disorder, aged between 20 and 44, have been released pending further inquiries.
Describing the events, the officer said: “I could hear screaming and shouting from the officers as they were being attacked – a call was made for vehicles to be used to assist officers.
“The only way to protect my colleagues was to place the vehicle across, side on, in front of them to offer protection.”
The officer said that glass bottles and rocks “as big as fists” were being used as missiles and “a number of people” tried to get into the van but it was locked.
Then, the officer said, they could smell burning so they escaped the vehicle.
“I was aware of the smell of burning and CCTV reported that people were attempting to set the vehicle tyres alight, with me still in it.”