An Post says no decisions have been made
Postmasters from around the country have protested outside the GPO in Dublin city.
They say they will resist any post office closures driven by An Post.
The Irish Postmasters Union (IPU) says it is to mobilise a national campaign to keep post offices open.
It comes as it is believed An Post is considering closing more than 200 outlets around the country.
The company says it is currently completing a detailed review of its network and no decisions have been made.
It expects the work to be completed within the next couple of months.
But the IPU is calling on politicians, communities and social organisations to give their support.
A number of postmasters attended the GPO protest, which the IPU said would not affect post office services.
While a more comprehensive Dáil rally is planned for April.
The IPU said all political parties had pledged their support to maintaining and supporting the post office Network at election time "and they must now live up to their promises."
General-Secretary Ned O'Hara says the completed plan, chaired by Bobby Kerr, needs to be implemented.
An Post CEO David McRedmond wrote directly to postmasters late last week about plans for a major re-organisation of the network.
The IPU claims this was done without their consultation. The IPU has handed in a letter at the GPO addressed to Mr McRedmond seeking an urgent meeting.
In a statement, the IPU says: "Postmasters will not support a 'solo run' by An Post.
"We fully understand that the role and services of the post office network need to change due to increasing use of technology, demographic changes and falling use of traditional mail services.
"The IPU understands that re-organisation and planning for the future is necessary, but we do not support major closures as part of this.
"We want to provide additional financial, State and social services and ensure that post offices continue to serve the economic and social needs of as many communities as possible.
"We believe that the key solutions we need are already contained within the Post Office Network Strategy Board report (chaired by businessman Bobby Kerr), which was completed before Christmas, includes a structured plan to sustain and develop the post office network over four years and earmarks a Government investment of €56m."
Postmasters presented half a million voter signatures to Government calling for greater action in 2015, and an online poll of 7,000 people in 2016 said 82% wanted Government to do more to support post offices.
Postmasters now marching inside the GPO against possible closure of rural post offices pic.twitter.com/3ivQx8BtPM— Sean Defoe (@SeanDefoe) March 14, 2017