Emergency patients were forced to evacuate Altnagelvin Hospital after a man brandished a gun at staff in the A&E ward.
Safety procedures at the Londonderry hospital were being reviewed today after staff discovered the patient with the weapon in the Accident and Emergency department.
More than 30 patients had to be evacuated from the hospital last night after the man began waving the handgun around.
Staff were able to isolate the man and evacuate some patients before police arrived.
When the PSNI arrived they were able to disarm the man and arrest him.
Head of Accident and Emergency at the Londonderry hospital, Alan McKinney, said the incident caused alarm among staff and patients.
“One of our patients in the waiting room noticed another patient waiting for treatment was carrying a gun openly. She was concerned about her safety and the safety of those around her and she alerted staff. Staff put the patient into an isolation space and evacuated departments around him,” he said.
Mr McKinney added that the man was behaving in an erratic way. “Staff felt there was a risk to everyone in the immediate area. Something like this has not happened in my memory. I have been at this department for 18 or 19 years and working in A &E as a job for 25/30 years. I do not remember ever having to evacuate for the safety of patients,” he told the BBC.
Mr McKinney added that it is difficult to assess all the risks A&E staff come across,
“The difficulty is we are trying to run a hospital. It would be easy to restrict access but we have got to have open doors.
“Staff deal with a lot of stress and strain during normal times, but to add to this the possibility of an armed attacker coming in just makes this completely unacceptable”.
The PSNI today said that the man arrested was in possession of a gun.
A spokeswoman said: “At approximately 8.45pm last night police received a report from staff at a hospital in Londonderry that one of the attending patients may be in possession of a gun. Police arrived and one man was arrested in A&E. A BB gun was also recovered. The man remains in custody at this time.”