More than 25,000 protesters have demanded that Health Secretary Jeremy Hunt rejects "insane" proposals to close a hospital's newly revamped A&E department.
Mr Hunt will decide by February 1 whether Lewisham Hospital in south-east London should replace its emergency department with an "urgent care" ward and turn its maternity services into a midwife-led unit.
Marching through Lewisham, campaigners said the "successful and well-run" hospital was in danger of being sacrificed to prop up a failing trust nearby.
The closures are part of a radical overhaul proposed by a special administrator in response to nearby South London Healthcare NHS Trust (SLHT) going into administration after it started losing around £1.3 million a week.
But SLHT - which runs three hospitals in the capital and was the first NHS trust to collapse - does not have responsibility for Lewisham Hospital.
Dr Louise Irvine, a local GP and chairman of the Save Lewisham Hospital campaign, said: "This decision is crazy and ill-thought-out... If Jeremy Hunt can close a good local hospital here, he can do it anywhere in the country - nowhere is safe."
Organisers said that between 25,000 and 30,000 protesters from all over the country had marched against the closure of the A&E ward, which recently completed a £12 million revamp. Millwall FC brought forward their FA Cup fourth round tie against Aston Villa to Friday night so the game would not clash.
The reorganisation proposed by special administrator Matthew Kershaw to deal with SLHT's collapse is intended to save around £42 million from the staff pay budget.
Previous estimates suggested those savings would include cutting 140 medical staff across the trust's three hospitals. Implementing the recommendations would cost around £313 million, while the trust's debts are expected to stand at £207 million by March.
Earlier this month, Mr Kershaw said SLHT remained the "biggest financial problem" across the NHS.
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