Thousands of protesters have marched through south-east London to demand that the government stops the planned closure of a hospital's accident and emergency department and downgrading of its maternity ward.

Organisers campaigning to save Lewisham hospital said the plans, which would see its emergency department replaced with an "urgent care" ward and its maternity services turned into a midwife-led unit, were "crazy and ill thought out".

The Save Lewisham Hospital group claimed up to 25,000 people took part in the march, though other estimates put the figure closer to 15,000.

The health secretary, Jeremy Hunt, is due to decide on the plans by 1 February.

The closures are part of an overhaul proposed by a special administrator after the nearby South London Healthcare NHS Trust (SLHT) went into administration as a result of it losing around £1.3m 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.

Campaigners said a "successful and well-run" hospital was in danger of being sacrificed due to the neighbouring trust's failure.

Dr Louise Irvine, a local GP and chair of the Save Lewisham Hospital campaign, said: "This decision is crazy and ill thought out. It is a big mistake and carries huge clinical risks of things going wrong for patients, but also political risk.

"If Jeremy Hunt can close a good local hospital here, he can do it anywhere in the country – nowhere is safe. This is very much a national issue, there are 60 hospital trusts across the country under threat of bankruptcy, many of them very good hospitals."