Drivers beeped their horns to show their support for protestors calling for a new hospital to be built.
Members of the King's Lynn Trades Council and East Anglia People Before Profit gathered in the rain outside the Queen Elizabeth Hospital (QEH).
The QEH is still in operation more than 10 years longer than its originally-intended working life and parts of its roof are failing.
But so far the 500-bed hospital, on the Gayton Road, has missed out on government funding for a new-build.
Jo Rust from the trades council said weekly protests were planned outside the QEH to show the strength of feeling in the community over the issue.
"What we need to do is sustain the depth of feeling," she said. "We won't have massive protests but we will have consistent protests."
One placard said the QEH was Britain's "most propped-up hospital". Another read: "Hospital on life support - condition critical."
The hospital's own risk register says: "There is a direct risk to life and safety of patients, visitors and staff due to the potential of catastrophic failure of the roof structure due to structural deficiencies."
Protestor Jordan Stokes said: "We need to get diggers on the ground now before someone gets hurt because of the defects with the building here."
Fellow activist John Hunt said: "The roof is caving in putting staff and patients at risk."
So far 194 props have been put in place to support the roof of the hospital which was built with an expected working life of 30 years but is still in use four decades later.
The hospital was awarded £20m to carry out urgent maintenance work last month.
Contractors are carrying out daily checks of the concrete planks used to build the structure. But members of the hospital board on Tuesday heard less than half of the roof had been surveyed.
While hospitals which were built in the same era from the same pre-fabricated components - like the James Paget, at Gorleston, and the West Suffolk, at Bury St Edmunds - were included in a list of 40 given the go-ahead for rebuilds last year, the QEH missed out on funding.
It is currently on a list of 16 from which eight will be chosen for replacement around 2030. An announcement is expected soon on how hospitals can bid for funding.
Almost 6,500 people have now signed the EDP's online petition calling for a new hospital.
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