Disabled patients picketed St Thomas' Hospital's open day on Saturday in protest at being denied patient transport.
Visitors arriving at the Westminster Bridge entrance were asked to sign a petition calling on the hospital to change its rules.
This, said the demonstrators, means that disabled and elderly patients do not have spare journeys left on their card for essential shopping trips let alone an occasional social outing.
The demonstration, which included a disabled woman living in Waterloo, was organised by the charity Transport for All on behalf of all patients in the catchment area.
"To deprive these people of their limited journeys is nothing short of a tax on disabled and older people by the hospital trust and an attempt to get local authorities who fund the scheme to pay for the hospital's transport" said a campaign spokesperson.
Guy's and St Thomas' NHS Foundation Trust told Disability Now magazine that it provides 170,000 free, non-emergency patient transport journeys every year.
A trust spokeswoman said patient transport was provided "on the basis of clinical and mobility needs" and access to other means of transport, and not on a patient's ability to pay.
The trust insists that its eligibility critieria for patient transport exceed Department of Health guidelines.
St Thomas' Hospital's open day on Saturday included music and entertainment as well as stalls with information on local health services. Visitors also enjoyed free admission to the Florence Nightingale Museum during the open day.
"This is my first open day since joining the trust, and I was hugely impressed," says
chief executive Ron Kerr. "Staff in all departments had really gone that extra mile in planning their stalls or opening their departments, coming up with very imaginative and entertaining ways to get visitors involved in their work."