Two people were taken to hospital after a stand 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 (21 May), 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 the area was evacuated by the emergency services and the ceremony, which involves a parade by the Household Division and Horse Guards, was cut short.
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.”
London Fire Brigade was also called to the scene and helped with the evacuation and checking stands for other areas of danger.
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 2 collapses with fear of more."
Footage shared to Twitter showed people walking through the Horse Guards Parade away from empty stands.
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.”