Non-urgent cases have been sent home after up to 10 ambulances were left queuing outside one hospital.
Few or no beds are free at Queen Elizabeth Hospital, King's Lynn, and the James Paget Hospital in Gorleston, both in Norfolk.
People with minor injuries have been urged not to attend A&E.
The alert status means plans designed to enable staff to cope with major incidents, such as terrorist attacks and train crashes, are put into action.
At the Norfolk and Norwich University Hospital (NNUH), which announced it had reached the emergency status on Wednesday afternoon, managers worked with other agencies to discharge non-urgent patients from the 1,000-bed hospital to create space.
However, the hospital said that between 60 and 70 beds were still being blocked because patients who were medically fit to leave had no where to go.
Paramedics treated patients in ambulances outside the NNUH on Wednesday evening.
'Diarrhoea and vomiting'
A spokesman for the East of England Ambulance Service said: "Most things we can manage - it only becomes critical to get someone out of the ambulance and into the hospital if someone is bleeding to death or in full cardiac arrest needing resuscitation.
"There were serious condition patients but they were being treated at the NNUH by hospital staff."
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."
The James Paget hospital and the Queen Elizabeth in Kings Lynn said there were sufficient beds, but a sudden change in the weather was putting extra pressure on them.
Norman Lamb, MP for North Norfolk, said the hospital was regularly well above the safe occupancy level of 85%.
He said: "If there was to be a flu epidemic this winter then we've got a major crisis.
"Across the country we're seeing occupancy rates increasing. There's also evidence that you run an increased risk of hospital acquired infections if you've got a hospital that's literally full.
"This is a serious problem and yet there appears to be nothing being done to increase the capacity to ensure that there's enough beds to treat patients."
Lack of staff
Milton Keynes and Hertfordshire health officials have said they are not on alert, but Bedford Hospital has been on red alert since Friday.
In Suffolk, Ipswich Hospital said it had very few beds but was managing the situation and a spokesman for the West Suffolk Hospital said its alert had been caused by sickness bugs.
In a statement Addenbrooke's Hospital said it had been on black alert, but by Thursday morning it was no longer on high alert, and a bed managing scheme had been implemented.