Three people have been taken to hospital after sections of the spectator stands collapsed during a Trooping the Colour rehearsal in central London.
A total of five people were treated for injuries following the incident in Horse Guards Road on Saturday morning, London Ambulance Service (LAS) 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.
But just before 11am, one of the stands gave way, resulting in three people sustaining injuries, one of whom was taken to hospital.
A short time later, part of another stand collapsed with two other people getting injured. One was taken to hospital.
Emergency authorities assisted those injured and evacuated the stand before the parade was cut short and the whole 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."
LAS said three patients were treated and discharged at the scene but transport to hospital was arranged for one of the patients later in the day.
A spokesperson said: "We were called at 10:59am to reports of an incident on Horse Guards Road, SW1.
"We sent a tactical response unit, an ambulance crew, two cycle responders and an incident response officer.
"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. We later arranged transport to hospital for one of these patients."
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."
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."
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 there had already been two collapses with fear of more.
"Was certainly a very 'British' experience with us queuing for an hour to watch marching horses and soldiers."