Protestors are expected outside Carmarthen's Post Office on Monday.
The King Street building is under threat of closure as moves are under way to look at relocating the branch from its large premises into a shop or business within the town.
No decision has been made yet and a public consultation will be carried out ahead of any move.
The Post Office said none of the 10 staff employed there are facing compulsory redundancy.
The branch is one of 70 flagship post offices which could convert into franchises based in shops or other retail businesses as part of a £1.3 billion reorganisation.
However, the threat has rocked the town with town leaders claiming the post office is the "jewel in the crown" of King Street — accounting for much of the footfall and main reason for shoppers to come into the street.
Mayor Phil Grice said the town must fight the and the town council was expected to hold talks over it last night.
A Post Office spokesman said : "Carmarthen has been identified as a branch that could potentially be partnered with a retailer – with any change being subject to a six week public consultation to gather the views of the local community.
"We are committed to having a Post Office branch within the area."
Post Office Communication Workers Union (CWU) general secretary Billy Hayes called the move a "partial destruction of the Crown network" and raised concerns about access to services and survival of the post office network.
He added: "This is a huge blow to the network.
"It's a particularly high proportion in Wales and we fear that towns will feel the loss of the Post Office very keenly.
"This move will have a huge impact on the high streets of small towns earmarked to lose their Crown Post Office.
These offices provide a dedicated specialist service to communities which will not be replicated by a window or two in a bigger shop.
"It leaves huge questions about the future of the Post Office – how can it realistically deliver services for passport applications, identity services and a range of financial services while being dramatically pruned back?."
The Carmarthen Journal will take an in-depth look at the issue in Wednesday's paper.