Peter Walker, chairman of the Priory Park Preservation Society, said: "This scheme, if it goes ahead, will be built on park land, no matter how vociferously councillors deny this.
"They have simply not done their homework adequately and they have avoided a precise description of what constitutes Priory Park."
He added that there were a total of four title deeds which gave the definition of the extent of Priory Park, with the last being after the death of the benefactor R A Jones.
The particular piece of land involved in the fourth of these was one which the council proposed to use for the new railway bridge.
At the moment, it is amenity space outside the park boundary - byt Mr Walker said it was still covered by the title deed.
"This piece of land includes 62 trees, almost half of them large oaks, which would almost all be felled under the council's scheme.
"These are not street trees, but park trees and this is an example of our elected representitives misleading the public."
In addition, the latest plans involved moving the footpath to the south of Priory Crescent.
Mr Walker addded: "At the moment, it is outside the park fence, but the plan is to place it inside the park, thereby concreting over what is currently a grass area. It makes you want to ask what kind of fantasy world our councillors live in if they can argue this scheme wil not take park land?
"They may delude each other with their talk, but there will not be many residents of the borough who will be convinced by this nonsense."
Southend council director of technical services David Watts siad at a meeting of the council's Integrated Transport Forum the status of the land affected by the railway bridge was a "matter of opinion".