Two people have been taken to hospital.
The incident happened at around 11 am at the Trooping the Colour event at Horse Guards Parade in London.
Up to 1,400 soldiers from the Household Division and the King’s Troop Royal Horse Artillery were taking part in the parade, and 400 musicians from the Massed Bands.
It is very similar to the Trooping the Colour event reviewed annually by the Queen – she is scheduled to do so on 2 June as part of the Jubilee celebrations.
An London Ambulance Service spokesperson said: “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.”