Extra staff had to be drafted in and more beds opened to try and relieve the pressure on the unit.
According to the Belfast Trust, 42 people were waiting on trolleys at one point.
From 8 pm, trolleys from the A&E department were backed up into the X-ray department as patients flooded in.
Such was the pressure on the Royal that ambulances were re-routed to the Ulster Hospital in Dundonald, east Belfast.
The Belfast Trust said in a statement: “The emergency department at the Royal Victoria Hospital experienced extreme pressures and as a result we declared it a major incident.
“We use this to call in extra staff at short notice to deal with particularly difficult situations.”
In November 2011, Belfast City Hospital’s Accident and Emergency Unit closed its doors and the Royal became the main hub of emergency care in Belfast.