US flags were torn down from the memorial in Endcliffe Park, Sheffield, which is dedicated to 10 American soldiers who died when the B-17 Flying Fortress, known as Mi Amigo, crashed in the park on February 22 1944.
Pensioner Tony Foulds, who looks after the memorial, urged vandals to stay away in a message posted on Twitter.
In a video posted by Sheffield councillor Shaffaq Mohammed, Mr Foulds said: “I’ve had a lot of damage done and my flags torn off and chucked onto the floor. Please, whatever happens don’t come near the memorial. For my sake, as well as these lads.”
Mr Foulds was eight when the badly-damaged bomber limped over the roofs of nearby houses and crashed into a wooded area, apparently to avoid him and his friends who were fighting on the grass.
He has since dedicated his life to the crew, believing he was responsible for their deaths.
Police said the damage had been caused on Tuesday, when warm weather brought people out to the park to socialise under the new rule of six in England.
Mr Foulds discovered the damage when he visited the memorial on Wednesday.
It comes two years after thousands attended a flypast arranged to mark the 75th anniversary of the crash, following a campaign by BBC Breakfast’s Dan Walker, who bumped into Mr Foulds while walking his dog in the park in 2019.
South Yorkshire Police superintendent Simon Verrall said in a statement: “Throughout the afternoon and evening of March 30, officers were required to respond to a number of reports of anti-social behaviour and damage being caused to the park.
“Sadly, one of the park’s memorials was also damaged during the evening, which has left volunteers deeply distressed.
“This activity is not acceptable and will not be tolerated in our open spaces. We are currently reviewing CCTV and mobile phone footage to identify those involved with a view to taking further action.”