The rally on October 18, which will culminate in a mass lobbying of Westminster, has been organised by the National Federation of Subpostmasters. The Federation will also deliver a petition, expected to carry more than three million signatures in favour of POCA, to Downing Street.
It says the petition highlights public support for the post office network, 97% of which is run privately by subpostmasters. Federation chiefs are also confident an Early Day Motion signed by 388 MPs will force ministers into a U-turn.
"We are confident our members will turn out in their thousands," said National President Sally Reeves. "The loss of the card account and demise of the network in general, with the loss of services such as TV licence payments, is something which affects thousands of businesses across the country.
"We have been warning the government about these issues for quite a while, but now we have unprecedented support in the House and more than three million petition signatures to support our views. The government needs to know the strength of feeling there is out there."
Julie Fox, who owns Overmoigne Post Office and Stores in Dorset, said the government had to act quickly to secure the future of the network. She added: "I have a customer who is 100 and for her the card account is vital. It's a hugely important issue. If we lose anything else both the shop and post office will no longer be viable."