Royal Victoria Hospital: Beds opened and extra staff called in
Extra staff were brought in to Belfast's largest emergency department to deal with the number of people seeking treatment on Monday.
The Belfast Health Trust triggered "escalation plans", that also included additional beds being opened at the Royal Victoria Hospital (RVH).
At one point, more than 100 people were waiting in A&E, and about 30 others were on trolleys.
At 17:00 GMT, the trust said there were no plans to call a major incident.
However, the trust told the BBC it was "monitoring the situation very closely on an almost hourly basis".
The emergency departments at the Ulster Hospital in Dundonald and Antrim Area Hospital have also been very busy.
According to a source, some patients had been waiting at the RVH since about 22:00 GMT on Sunday.
The trust confirmed that in the previous 24 hours there had been pressure on all emergency departments across Northern Ireland.
A major incident was declared at the RVH last month, when a large backlog of patients led to lengthy waiting times in its emergency department. At that stage, 42 people were waiting on trolleys.
BBC Northern Ireland's health correspondent Marie-Louise Connolly said: "The BBC understands that tonight several senior managers are in the department monitoring what is happening.
"Considering the negative publicity after January's incident, it's understood that staff have been told only to declare another one if there is no other option."
By 20:00 GMT, 80 patients were waiting in the RVH's emergency department, 15 on trolleys. One patient had been waiting from 02:00 GMT.
Ray Rafferty of the union, Unison, said there had been an excessive number of patients and at one point, the department ran out of trolleys.
"The difficulty was the patients that were left over from Sunday," he said.
"It was an exceptionally busy weekend, at not only the RVH but most hospitals, and it took time to process those patients.
"Unfortunately, some of them were still waiting today whenever the patients that normally arrive on a Monday arrived today, which made the situation untenable."
Chair of the health committee Sinn Féin's Maeve McLaughlin said the initiation of an "escalation measure" highlighted the pressure that existed within the system.
She said: "For the past 18 months we have stated that the current situation is symptomatic of the overall downgrading of A&Es across our hospitals and the need to address, not just the closure of A&E departments, but also where these departments feed to within our hospitals.
"The review that the minister has announced must be fast-tracked. In the interim, he must introduce measures to ensure we do not have a major incident called again in the coming weeks."
Earlier this month, the Belfast Health Trust admitted that delays in treatment may have been a significant factor in the the deaths of five patients at the RHV in 2013.