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 taken away 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 they worked alongside St John Ambulance volunteers to treat five people 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 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."

Australian Associated Press