On social media, the Trust confirmed the hospital was at “full capacity” and under “extreme pressure”.
On Sunday morning they issued a plea for people not to attend “in any circumstances” and said the situation was being “continuously evaluated”.
In an updated statement it was confirmed the situation remains “extreme” and said the scenario of having to close the emergency department to patients “was inevitable”.
Reacting to the announcement Alliance Party MLA Danny Donnelly said: “This is incredibly worrying. A hospital A&E working at full capacity and cannot accept any more patients means staff working under extreme pressures for long periods.
"This is unsustainable and urgently needs to be addressed. We are in a healthcare crisis and it’s only Nov.”
Sinn Fein’s Declan Kearney said he has been left “deeply concerned” by the situation.
The MLA said: “I urge the public to continue to co-operate with hospital staff and people should definitely attend the ED if their condition is urgent or life threatening.
“Unfortunately the knock-on impact of last night’s closure will be that those who attend with conditions which are not urgent may endure a long wait to be seen.
“This latest incident in our hospitals is deeply concerning with pressures increasing as we move into winter.
“Health service staff and patients need an Executive up and running, to put one billion pounds extra investment into health, to tackle waiting lists, recruit more doctors and nurses, and fund cancer and mental health services."
A spokesperson for the Northern Trust said: “We have warned for some time that it was inevitable that one or more hospitals could be forced into a situation where they had to close their emergency department for safety reasons due to the number of people in the department with high numbers needing to be admitted.
"On Saturday night, Antrim Area Hospital had a disproportionately high number of critically ill patients, a number of whom arrived in quick succession.
"A clinical decision was made by the senior emergency department consultant that the department could no longer operate safely and could not respond appropriately to any further critically ill patients who might have arrived.
"As such a major incident was declared. Temporary closure of the emergency department allowed the situation to be managed and controlled in a safe manner.
"This was a regrettable position to be in, but in the circumstances it was the only safe and appropriate response. The Trust is continuing to work closely with the Northern Ireland Ambulance Service and other trusts to coordinate an appropriate response given regional pressures.
“The Antrim site currently has a high number of very ill patients who require admission into the hospital and therefore our priority today has to be patient discharge.
"We would appeal for the full co-operation of patients, families and carers in assisting us with discharging loved ones who are ready to go home, to help free up much needed beds and reduce the overall pressure on our service.
“People should definitely attend the emergency department if their condition is urgent or life threatening. Patients will always be seen in order of clinical priority, therefore those who attend with conditions which are not urgent will endure a long wait to be seen.
“We thank the public and our staff for their patience and understanding in these difficult circumstances.”