Postmasters are calling for urgent government action to safeguard the network of rural post offices.
The number of post offices fell by more than 4,000 since 1999
A four-million-name petition is being handed into Downing Street, and the National Federation of Subpostmasters is holding a rally in London.
It says income lost from some services may threaten the network's future.
The post office network is losing £200m a year and the government is widely expected to require some closures as part of a subsidy renewal agreement.
The number of post offices has fallen from 18,393 in 1999 to 14,376 in 2005, according to Postwatch, the postal services watchdog.
The government says the 800 smallest post offices are used by an average of 16 people a week, and that the rural network is currently supported by an annual subsidy of £150m.
A Downing Street spokesman said the government had made an unprecedented investment in the Post Office network of more than £2 billion, including £750 million for the rural network.
"We recognise that some offices, maybe, will never be commercially viable but play an important social role. Equally, however, we have to make sure that the network is sustainable," he said.
Trade and Industry Secretary Alistair Darling said doing nothing was not "sustainable" and the Post Office needed to adapt.
He said that many of the services once offered exclusively could now be done elsewhere - for example, people were now able to buy road tax online, 98% of new pensions were paid straight into people's bank accounts, and TV licences could no longer be bought at post offices.
It was "inconceivable" there would be no subsidy of post offices, but the amount would depend on the final decision on the future shape and size of the network, he said.
"I want to maintain a national network, but I want to do it in a way that will last so we don't keep coming back again and again to the underlying problems we have had in the past few years," he said.
The NFSP says the petition is Britain's biggest-ever domestic petition handed to Downing Street.
General secretary Colin Baker said it was "crunch time" for post offices.
"The 28 million customers who use the Post Office every week are confused as to what the government wants of the network and of the people who run it," he said.
"We believe that the time is now overdue for ministers to decide."
POST OFFICES IN CRISIS
Despite subsidies of £150m, post offices lost £111m in 2005
Postcomm estimates 1,500 of 8,000 rural post offices make money
72% of subpostmasters are uncertain about the future, says Postcomm
Revenue from services withdrawn from post offices by the government has fallen £168m in the past year
Among problems faced by post office income is the planned withdrawal of the Post Office card account, used by millions to access pensions and benefits.
Almost 400 MPs have signed an early day motion calling for that decision to be reversed.
It was tabled by Kate Hoey MP, who chairs the All-Party Parliamentary Group for sub post offices.
The NFSP campaign is also backed by Citizens Advice, which said the closure of many post offices had already hit pensioners and disabled people particularly hard.
"The government should recognise the role of the post office at the heart of the community and invest in the future of the post office network."
The Transport and General Workers Union called on the government to consider the consequences of fewer rural post offices.
"Fine words butter no parsnips and we need the government to clearly understand the balance sheet approach to rural services is not the right one," said Chris Kaufman, its national secretary for agriculture.