Two people were taken to hospital after sections of the spectator stands collapsed during a Trooping the Colour rehearsal in central London.
Three others were treated at the scene by paramedics following the incident in Horse Guards Road at about 11am on Saturday, London Ambulance Service said.
Dozens had turned up to watch the event, which is the Major General’s review of the parade ahead of the full event on June 2 for the Queen’s Jubilee celebrations.
Witnesses said crowds were evacuated by the emergency services and the ceremony, which involves a parade by the Household Division and Horse Guards, was cut short.
Emergency authorities assisted those injured and evacuated the stand before the parade was cut short and the area was evacuated.
An Army spokesperson said: “During the Major General’s Review on Horse Guards Parade, two parts of the viewing stand unfortunately gave way, resulting in a small number of injuries.
“The emergency services were called to assist those who were injured and the remaining crowds were safely evacuated from the stands.
“Safety is our number one priority and we are urgently working with our partners and relevant organisations to understand what happened and make sure it doesn’t happen again.”
A London Ambulance Service spokesman said: “We worked alongside St John Ambulance volunteers to treat five people at the scene.
“We took one patient to a major trauma centre as a priority. St John Ambulance took a second patient to a major trauma centre as a priority. We discharged three patients at the scene.”
Alex O’Grady, 34, from Bromley, told the PA news agency that the incident was “surreal”, a “terrible state of affairs” and that “people were in tears at the scene”.
He said: “We heard a big bang - thought someone had fallen down steps initially, turned to see a hole in floor on second row from front of stand A.”
Mr O’Grady said he saw a woman was shouting over the hole at a man, whom he presumed was her husband, but there was no response and paramedics soon started to treat the man.
He added: “There was also a young child who had a scuffed knee, another man obviously concerned and the wife was inconsolable.”
One Twitter user wrote: “Was fun experience to watch until we stood up for the National Anthem, the stand next to us collapsed with someone falling through.”
“They cut the proceedings short and police began evacuating us for safety concerns as ther e had already been 2 collapses with fear of more.
“Was certainly a very ‘British’ experience with us queuing for an hour to watch marching horses and soldiers.”
Another witness told the PA news agency: “There was a loud bang then a scream and a voice shouting ‘help, help.’
“A number of ambulances were at that location and stretchers were brought out for the casualties.”
Footage shared to Twitter showed people walking through the Horse Guards Parade away from empty stands.
The Ministry of Defence said the situation was ongoing.
In a statement, a spokesman said: “We are supporting the emergency services following an incident on the Horse Guards parade square.
“It would be inappropriate to comment further at this time.”
