A woman is feared to have been injured after falling through a stand during a Trooping the Colour event.
Crowds of tourists and locals who had turned up for the event were evacuated by the emergency services as a result of the incident.
Reports from live at the scene suggest that proceedings, being reviewed by The Major General, were interrupted, the Mirror reports.
One witness said: "At Trooping the colour, part of stand, where spectators sat, has partially collapsed. No news of injuries."
Zoe Clark-Coates added: "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 person described the situation as "chaos."
She explained: "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.
"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."
Today's event is almost identical to Trooping the Colour reviewed by the Queen.
However, it excludes the Guardsmen Street Lining on the Mall and extra mounted who officers ride when the Queen is present.
Up to 1,450 soldiers of the Household Division and The King’s Troop Royal Horse Artillery took part in the parade on Horse Guards.
The soldiers were inspected by Major General Christopher Ghika CBE, Major General Commanding the Household Division.
They were also joined by 400 musicians from the Massed Bands, for the first of two formal Reviews.
Next weekend Prince William will attend, before the Queen oversees the main event as part of the Jubilee bank holiday weekend.
An Army spokesperson 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.”
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