Priory Park will not be touched as part of the proposal but the council intends to use the existing highway and land owned by business to widen the road.
Trees along Priory Crescent may have been felled but the council insists two for each one lost will be planted.
The Priory Park Preservation Society is urging its members to write to councillors and also send a copy to the council's technical services department.
The society will hold a demonstration outside the gates of the park on Saturday starting at 10:30am to continue its campaign.
Peter Walker, chairman of the Priory Park Preservation Society, said he was convinced the council had a "hidden agenda" behind the controversial proposals.
However, Roger Weaver, Southend executive councillor for transport, dismissed the suggestion as speculation.
Mr Walker said: "One has to ask why the council wants to build a road which does absolutely nothing to ease congestion and which can only add to Priory Crescent's pollution problems.
"In the absense of firm information one can only guess, but clues have been given by unwary councillors who have let slip revealing answers to questions asked."
He claimed the council wanted to see huge amounts of land to the east of the town developed, including Thorpe Hall Golf Club and former MoD land at Shoebury. That would mean more traffic crossing the town from east to west, using existing roads as the idea of a road to the east from the north had been rejected by Essex County Council and Rochford District Council.
Mr Walker said many people believed Southend had reached its limit in development.
"Those of us fighting this road scheme are not prepared to see our quality of life sacrificed for some spurious notion that we, Southend, can compete economically with better serviced inland towns.
"Because we accept Southend can't compete, we believe any development should be for the benefit of existing residents or to enhance the natural features and advantages the town has.
"Priory Park, for one, must compare favourably with any urban park in neighbouring towns; the seafront is our most obvious asset; and what about trying to improve the public transport system so buses actually take people to places they want to go to, when they want to go there?"
Mr Weaver (Con, Eastwood Park) said the claims were "pure speculation" and added: "We are not aware of any plans for development at Thorpe Hall Golf Club or of other proposals.
"Priory Crescent was first identified as a road under stress in 1994 and then again in the local transport plan last year.
"The flights of imagination by these protest groups seem to go beyond all reason."
Mr Weaver said the latest proposals were out to public consultation before a final decision is made by the council.