A doctor, a security guard and a porter were injured during the incident in the Accident and Emergency department of Craigavon Area Hospital on Wednesday.
It began after a male patient was asked to stop shouting at his girlfriend, but became verbally abusive to staff.
He was then joined by a group of other youths in the hospital foyer and a security guard was punched about the head.
The brawl continued just outside the hospital and the youths began throwing stones at staff, hitting a porter and a doctor.
Dr Gareth Hampton, the senior house officer who was injured, said it was a very serious incident for the hospital.
"It was a serious incident involving a lot of people, a very high level of violence," he said.
'Reprehensible attack'
"Both the security staff, the nursing staff and all the medical staff were very exposed having an actual patient becoming so violent and aggressive."
The police are studying a security video of the incident and said they were looking for one person in connection with the incident.
Sinn Fein councillor John O'Dowd condemned the attack as "reprehensible".
"Health service and other ancillary staff are under enough pressure because of government policies without having to endure this type of mindless and sickening behaviour," he said.
"I totally condemn this behaviour on people trying who are doing their best, quite often under difficult circumstances, to provide a caring and professional standard of service for the whole community."