Demonstrators gathered with placards in a show of anger at plans to axe a town centre post office in Eastbourne.
They hope to force Post Office bosses into a rethink over the closure of the Upperton Road branch.
Post Office chiefs have said the branch, one of the busiest in the resort, needs to close in November to stem losses of more than £65,000 a year.
Users say closing it would leave an "appalling hole" in the community and they gathered outside the branch to highlight their anger.
A petition has been raised and campaigners have collected dozens of signatures.
The decision to close the post office has drawn cross-party opposition on Eastbourne Borough Council, with Liberal Democrat leader Beryl Healy saying she was shocked by the move.
A letter from council chief executive Martin Ray has been sent to Post Office bosses in the hope of gaining an extension to the four-week consultation period.
Campaigners are urging people in Eastbourne to bombard the Post Office with letters of objection in a bid to force a U-turn.
Liberal Democrat parliamentary spokesman Stephen Lloyd said: "I find it incredible the Post Office is seriously planning to close this post office. The inconvenience for businesses and residents would be unacceptable. We must do everything we can to stop it happening."
Upperton Road is the second post office to close in Eastbourne after the Whitley Road branch.
Mother-of-two Karen Allen, 36, of Upperton Road, Eastbourne, said: "Two big post office closures in a short space of time is simply not on.
"Many pensioners, mothers and other people without transport rely on having a post office nearby. Great inconvenience will be caused."
Kevin Ray, head of area for the Post Office, said two other branches were within half a mile of Upperton Road - in Terminus Road and Ocklynge Road, Old Town.
Major improvements to expand the Terminus Road branch would go ahead should the Upperton Road post office be axed. More counters would be added to cope with extra customers, along with car tax facilities, a bureau de change and a post shop.
The Old Town branch would undergo a slight overhaul, with car tax facilities being added and increased staffing.
Mr Ray said: "We simply cannot continue to operate a branch that costs nearly £65,000 more to run than it contributes each year. The Post Office is losing £1.7 million annually.
"The full impact of switching benefit payments from books into bank accounts is likely to eventually cut as much as £400 million more a year off our revenue.
"The Government wants another £50 million annually trimmed off operating costs of our directly-managed branches so closure of this Eastbourne post office is the logical option."
Some Upperton Road staff will transfer to Terminus Road. Mr Ray said: "We have a long-established record of supporting our people and we would do everything we can to ensure their preferences are met."
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