A hospital where up to 10 ambulances had to queue because doctors had run out of beds has scaled down the alert.
Ten ambulances queued at the peak of the problem
Paramedics were forced to treat patients in the back of the vehicles as they waited near the Norfolk and Norwich University Hospital (NNUH).
Managers worked with other agencies to discharge non-urgent patients from the 1,000-bed hospital to create space.
The alert was scaled down after three hours with the hospital operating normally throughout the evening.
At the peak of the emergency, the hospital said 10 ambulances were waiting to admit patients.
Norfolk's two other general hospitals, the Queen Elizabeth Hospital in King's Lynn and the James Paget Hospital in Gorleston were also on the top level of alert - meaning all contingency measures were exhausted.
And a spokesman for Addenbrooke's Hospital in Cambridge said it had also faced similar problems - which is termed "black alert".
He said: "We are not on black alert currently, but we have been for the majority of the day.
"We are aware of the pressure on our neighbouring trusts and the impact that may have on us."
NNUH spokesman Andrew Stronach said there was no single incident that brought on the beds crisis.
"It's just general run of the mill problems, like chest pains, collapses, diarrhoea and vomiting."
Earlier the hospital said it was working with social services and other organisations to help move patients with less-severe medical conditions.
Amber alert: Early warning of pressure increase in the normal activity.
Red alert: Escalating pressure in one or more part of the system continuing to increase.
Black alert: Contingency measure are exhausted and pressures are not sustainable.
"As a result of the extreme pressure we are under, we have declared a major incident in order that all the agencies can take appropriate action to discharge patients and free-up beds," the statement added.
It urged people with minor injuries not to attend the A&E.
Matthew Ware, from the East of England Ambulance Service, said: "At its peak we had 10 ambulances outside the hospital and we've probably only got about 20 to 25 vehicles on the road in Norfolk at any one time, so that takes out quite a large proportion of our fleet.
"We're not talking about patients who are having cardiac arrests and that type of thing, they're patients that can be cared for safely in that environment but certainly we need to be able to get our vehicles back out there on the road."
Concerns over GPs
Adrian Ing, the Norfolk representative of the Royal College of Nursing, said he was concerned that more than a third of all ambulances typically on duty in Norfolk were queued up at the Norfolk and Norwich University Hospital.
He added was also concerned at cuts in community hospital beds and changes to the GP out of hours service.
"It's not just a case of bed blocking," he added.
"It's also about how people are using the out of hours service, which is now being run by the East of England Ambulance service instead of GPs.
"Are they using it or are they pitching up at the A&E at the Norfolk and Norwich?
"If they are not going to have access to a GP through the way they used to, they may just go to the hospital and wait to be seen."