Two people were taken to hospital after a stand reportedly collapsed when spectators rose for the national anthem during a trooping the colour rehearsal in central Londres.
Three others were treated at the scene by paramedics after 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 before the full event on 2 June 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.
A London ambulance service spokesperson 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.”
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.
“Was certainly a very ‘British’ experience with us queueing for an hour to watch marching horses and soldiers.”
Footage shared on Twitter showed people walking through Horse Guards Parade away from empty stands.
A Ministry of Defence spokesperson said: “We are supporting the emergency services following an incident on the Horse Guards Parade square.”