A video shows the policeman arrest the man before later admitting he was "perfectly innocent".
A jury is shown a video of married PC Timothy Brehmer "crying hysterically" after killing his lover.
Hampshire Police have joined an operation using the public's video to take action against dangerous drivers.
Gaia Pope's body was found 11 days after she went missing in 2017.
Police and council officers are patrolling the seafront in Bournemouth and Poole as thousands arrive.
- Copyright: Matt Pinner
Dorset Police has urged people to stay away from Bournemouth Beach and other beaches in the county.
Assistant Chief Constable Sam de Reya said: "These are unprecedented times and we are urging people to stay away from the area of Bournemouth Beach and other Dorset beaches."
The force said it will be increasing patrols in the area to tackle anti-social behaviour.
ACC de Reya added: "We are also reliant on people taking personal responsibility and strongly advise members of the public to think twice before heading to the area.
"Clearly we are still in a public health crisis and such a significant volume of people heading to one area places a further strain on emergency services resources."