Most of the park will close when it hosts 2012 events
Residents formed a ring around part of Greenwich Park in protest at its selection as the venue for equestrian events at London's 2012 Olympics.
A temporary 23,000-seat arena will be built in the south-east London park for four weeks during the Games.
Residents group No to Greenwich Olympic Equestrian Events (Nogoe) fears it will cause permanent damage to the park.
The London Organising Committee for the Olympic Games (Locog) said trees would not be cut down for the arena.
Events including show jumping and dressage would be hosted at the venue.
'Small and fragile'
A Nogoe spokeswoman said: "Greenwich Park is not the right choice for the equestrian events.
"The park - a world heritage site - is too small and fragile. Permanent damage could be caused to the ecology, archaeology and wildlife."
She said it would also be "socially and morally wrong" for Olympic organisers to close the park during the Games.
Locog equestrian manager Tim Haddaway said the park would be closed for a minimal period.
He said: "Even within that four-week period the children's playground and areas of the flower garden will remain open for all bar just one day of competition."
Mr Haddaway added: "Some people are concerned that we are going to cut down trees and damage the ground irreparably. It's simply not the case - there is no need to cut down trees for the Games."
He said the Games would introduce more Londoners to horse riding and there were plans to create a new children's playground at the park after it hosted the events in 2012.