The Protect Our Post Offices campaign, organised by the National Federation of SubPostmasters (NFSP) aims to highlight what it says is a growing crisis in the network.
It is urging MPs to take immediate action to safeguard the future of the UK's post offices.
Sub-postmasters will be asking customers to sign special campaign postcards, and the NFSP expects hundreds of thousands of these postcards to be sent to MPs urging them to call on the government to halt the sale of Royal Mail and retain a stake in the company.
They also want to ensure people can continue to collect their pensions and benefits at post offices and that post offices maintain a comprehensive range of banking services.
George Thomson, General Secretary of the NFSP, said: "The government and Post Office Ltd (POL) continue to insist that all is well for the UK's post offices, but we know that the reality is very different. Sub-postmasters are finding it increasingly difficult to make a living from their post office; times have never been harder and urgent action is required to ensure this vital service remains in communities across the UK."
The UK Government announced plans for the Royal Mail sell-off to go ahead in October. Campaigners say the minimum they want is for government to retain a stake in the business.
Sub-postmasters are increasingly dependent on Royal Mail income. Around 33% of their pay is generated by the sale of Royal Mail products. Campaigners say that they fear post-privatisation, sub-postmasters' pay could see further cuts as Royal Mail's new owners seek to maximise their profits.
Mr Thomson said: "We have reached a critical point in the future of the post office network; doing nothing is not an option as the current situation is unsustainable."