Two people have been taken to hospital after part of a stand collapsed during a rehearsal for Trooping the Colour.
Five people were treated following the incident on Saturday as a crowd watched the event at Horse Guards Road in central London.
Witnesses described hearing a “loud bang and then a scream” and someone falling through the stand.
Two people were taken to a major trauma centre “as a priority”, London Ambulance Service said.
Three others were treated by paramedics and were discharged at the scene following the incident at about 11am.
Spectators were watching the rehearsal ahead of the full event on 2 June for the Queen’s Platinum 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.
Sam Miller wrote on Twitter that a stand close to him collapsed “with someone falling through” as people stood up for the national anthem.
Another witness said: “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.”
In a statement, the Ministry of Defence 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.”