Three people were taken to hospital when a section of the stands for spectators watching Trooping the Colour collapsed on Saturday.
A woman is understood to have been injured after falling through the stands, with others around her evacuated by emergency services.
But proceedings were suddenly interrupted when a part of the stand collapsed.
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. We later arranged transport to hospital for one of these patients."
Zoe Clark-Coates, the TV presenter and author who was among the crowd at the time, wrote on Twitter: "What we just witnessed was terrifying, but we are safe!
"Part of the floor in the stand collapsed, and at least one person fell through. We are praying for the injured and shocked. The stands are being evacuated."
Another witness described the situation as "chaos", telling The Mirror: "We were told to stand and heard a scream and then there was chaos in the section.
"Turned out part of it had collapsed and a woman had fallen through. The paramedics rushed in but started yelling 'we can’t find her'.
"She was extracted after several minutes and seemed fine but badly shaken and was put on a stretcher and loaded into an ambulance.
"The crowd was calm but was later moved out of that section. Then a second section had a similar collapse and the crowd was moved but with no one falling through."
She added: "The striking thing was that there seemed to be complete confusion with police about what to do and where to find the victim under the bleachers. But the medics did the best they could with no guidance."
One witness said the stand appeared to collapse when spectators stood for the national anthem.
Sam Miller wrote on Twitter: "Was fun experience to watch until we stood up for the national anthem, the stand next to us collapsed with someone falling through."
The full event will take place on June 2 for the Queen's Jubilee celebrations.
Saturday’s Trooping the Colour, reviewed by The Major General, is identical to Trooping the Colour reviewed by the Queen, with the exception that it excludes the Guardsmen standing on street lining duty along the Mall.
Up to 1,450 soldiers of the Household Division and The King’s Troop Royal Horse Artillery, along with up to 400 musicians from the Massed Bands, took part in the parade on Horse Guards Parade for the first of two formal reviews.
The soldiers were inspected by Major General Christopher Ghika CBE, who commands the Household Division.
Prince William will attend the event next weekend, before the Queen oversees the main event as part of the Jubilee bank holiday weekend.