The vandals are believed to have struck on Tuesday evening or early Wednesday morning.
The memorial benches on the bandstand side of the lake had been thrown into the lake. On the opposite 'boathouse' side of the lake the benches had all been turn upside down.
The benches have now been recovered from the lake by workers.
Elaine Smith, chairman of the Friends of Stanley Park said she is 'paranoid' over the vandalism happening in the park.
She said: "They are driving me daft. It's getting to the stage where I am seeing young people in the park and thinking is it them that is doing this. Some of the swings have been wrecked, they are starting fires and doors are being smashed as well. It has been going on all winter which is worrying as it is usually quiet."
Elaine explained how bolting down the benches would cost a 'lot of money' and time. She added: "I do wish the local schools would speak to their pupils and let them know the distress this sort of action can cause. I'm getting paranoid about the damage and it's not the right attitude to have at all.
"The park is for the future generations but some of them seem to want to destroy it."
Elizabeth Gomm, a member of the Friends of Stanley Park, has a memorial bench to her late partner in the park and said it was a shame to see the damage.
She said: "There is an ongoing problem of vandalism and damage to property in the park, which holds the title of Britain's Best Loved Park.
"If my bench, dedicated to my former partner had Mike Foster, had been turned over it had been righted by the time I got there. Probably by some kind person who knows I walk in the park most mornings.
"It took three men to retrieve each bench from the lake, they are so heavy, so it must take a group to throw them into the lake and over a considerable length of time. I know this is as distressing for the park management, staff, Friends and volunteers who love and cherish our beautiful park as much as those of us who use it.
"It is most distressing, of course, for those, like me, who bought a bench as a memorial to someone they loved and who find peace in using their benches or in seeing other people enjoying them. It is a shame a small minority of thoughtless people can cause such heartache.
"I have been assured that there are plans to secure the benches and that those damaged will be restored, but it will take time. Hopefully, the council will put resources into helping park management find ways of tackling the problem of vandalism and that there will be a police or security presence in the hours between dusk and dawn to protect our beautiful park."