A hospital in Cambridge has declared the "highest level of alert" after seeing "exceptional levels of acutely unwell patients" needing beds.
The trust which runs Addenbrooke's Hospital said it was temporarily diverting ambulances to other hospitals and all elective surgery was cancelled.
Some 32 people out of 50 in the A&E department on Tuesday morning needed inpatient beds, a spokesman said.
The hospital has urged people only to attend the department in an emergency.
Cambridge University Hospitals NHS Trust declared the "Opel Level Four" alert - the highest - early on Tuesday.
Patient Emma Overend, from Wellingborough in Northamptonshire, was ready to have a minor gastric operation when she was sent home at about 10:00 BST.
She arrived at about 07:00 and an anaesthetist told her "the hospital is on black alert".
"We were told everything was on hold. Surgeons were coming out to explain that operations couldn't go ahead," Ms Overend said.
Last week was extremely busy week in our A&E! pic.twitter.com/SvUej5sOYO— CUH (@CUH_NHS) March 26, 2018
Ms Overend, whose previous surgery at Luton Hospital in November was also cancelled, said she was "really sad about the state of our country and the NHS".
The hospital trust has apologised to patients and said staff were "taking action to ease pressures and create flow through the hospital".
A spokesman confirmed the hospital had "50 patients in our emergency department this morning, 32 of whom needed inpatient beds".
"So far, we have had 12 patients who have waited longer than 12 hours from the decision to admit," he said.
"Patients can help us at this time by only coming into A&E if it is an accident or emergency and by not bringing large numbers of family or friends into the department where seating is at a premium."