A major incident was declared at Belfast's Royal Victoria Hospital hospital on Wednesday night due to a backlog of patients.
Additional staff had to be drafted in while extra beds were also opened in order to ease pressure.
It's understood around 40 people had been left waiting on trolleys at one stage on Wednesday.
It's believed the difficulties were finally brought under control at around midnight.
A spokeswoman for the Belfast Health Trust said the emergency department at the Royal "experienced extreme pressures and as a result we declared it a major incident".
"Belfast Trust's emergency department at RVH experienced extreme pressures yesterday and as a result we declared it a major incident," she said.
"This is a mechanism we use to call in extra staff at short notice to deal with particularly difficult situations.
"Our staff dealt professionally and quickly with a difficult situation and within a few hours had rectified the situation."
The Trust said the needs of patients were "put first" amid the problems.
"We would like to pay tribute to the staff who work in such a challenging and demanding environment," she said.
Following the situation, dozens gathered outside the RVH on Thursday afternoon in protest against what it sees at the critical state of services at the Belfast hospital.
Responding today, Health Minister Edwin Poots - who was booed as he visited the Royal on Thursday - said last night's difficulties "were exceptional".
"The additional staff who responded manned other areas which were then used as additional wards to address the unexpected demand," he said.
"Last night's circumstances were exceptional and it is important not to confuse an exceptional circumstance with overall performance in Belfast where waiting times have been demonstrating some improvement."
"They work under significant pressure and at all times put the needs of our patients first. While we do not want anyone to wait, and we fully understand how frustrating that can be for patients and the staff looking after them, all steps were taken to ensure patients were safely cared for."
Speaking to the BBC's Nolan Show, Janice Smyth from the Royal College of Nursing described Wednesday night's problems as "totally unacceptable".
"The pressure on that department has been building," she said.
"We are really concerned that the situation was allowed to develop to the point that we had to implement a major incident to deal with it."
Ulster Unionist Party health spokesman Roy Beggs blamed recent "ill thought out" decisions to reduce services and an "abdication" of leadership by Mr Edwin Poots.
The problems on Wednesday have led to further concerns over the reduction of accident and emergency services at other hospitals - including Lagan Valley.
DUP MLA for Lagan, Jeffrey Donaldson told the Belfast Telegraph he had been in contact with Health Minister - and party colleague - Edwin Poots, over accelerating plans to "integrate" out-of-hours GP services with A&E at Lagan Valley to ensure "round-the-clock" care.
"When the South Eastern Trust decided to close the Downe and Lagan A&Es at weekends, I warned the results would be added pressure to the acute services at the RVH," he said.
"And, on the first weekend of this new policy I understand the department was under enormous pressure, coming to a head with a major incident on Wednesday.
"This cannot continue, where local hospitals are closed at night."
SDLP councillor for Lisburn Pat Catney said the serious problems raised further concerns for residents in his area, after emergency services at the Lagan Valley Hospital were stopped after 8pm, and will be closed at weekends.
The reduced hours came into effect on January 4.
Mr Catney said he had been inundated with calls from residents.
"People are really concerned. I am really concerned. People want to know what is going to happen to them if they get ill after 9pm on a weeknight, or at the weekend, and they have to go to the Royal Victoria Hospital," he said.
"Will they be left lying on a trolley like those people last night? How many of those patients were from Lisburn?"
SDLP MLA for South Belfast Fearghal McKinney said he had "warned" Health Minister Edwin Poots that plans to reduce service hours across A&Es would have a "detrimental impact on those hospitals which would be left to deal with the service deficit".
"Now we’re seeing critical pressure placed on Craigavon Area Hospital and Belfast RVH, which are experiencing unprecedented demand as a result of these flawed decisions."
The Unison union - which held a protest outside the Royal today - claimed the recent closure of the emergency departments at weekends would "further compound the crisis".
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