Police released a video of protesters setting fire to a force van while an officer was still in the vehicle.
The officer described hearing “screaming and shouting” as he tried to protect colleagues during the protests in Bristol.
The Kill the Bill rally was organised in opposition to proposals to increase powers for police at protests.
The issue gained traction after police were criticised for a heavy-handed operation to clear a Sarah Everard memorial and protest, where police used lockdown powers to clear the site in Clapham Common, a park in south-west London.
On Thursday, police released a short video of the attack on the van with the officer inside.
“I was aware from the radio that my colleagues were under attack outside the front of Bridewell police station where damage had been caused,” 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.
"There were glass bottles, rocks as big as fists, pallets, fences and other objects strewn across the road and being used against the police."
The officer said that several people tried to get inside the van but its doors were locked.
“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, and so I began to prepare to decamp the vehicle.”
The government's Police, Crime, Sentencing and Courts Bill, which is still before parliament, would give police new powers to impose time and noise limits on street protests.
That angered activists, particularly after the Everard vigil on March 13.
The body of Everard, 33, a marketing executive, was found in woodland in Kent after she went missing while walking home from a friend's house on March 3.
Although it is yet to be passed, the government bill was introduced before the Everard case and covers a wide range of matters, including the policing of protests.
But the two became connected in many people's minds because the bill was up for debate in parliament two days after the London vigil.
