The commanding officer says the horses were “spooked by some construction works on a quiet side road in Belgravia where building materials were dropped from height right next to them”.
Military horses have caused “total mayhem” after bolting during exercises and charging through central London in rush hour.
The Household Cavalry said the animals were spooked by builders dropping rubble from a height “right next to them”.
The incident happened on Wednesday morning as the horses took part in what the army called “routine exercise” in the Belgravia area.
Seven horses and six Household Cavalry soldiers were on the exercise when chaos erupted.
Four service personnel were thrown off and five horses got loose – with two of them filmed galloping through the streets.
A white horse was seen covered in blood after hitting a taxi, while another animal smashed the windscreen of a parked tour bus.
The pair were eventually captured by police near Limehouse, five miles from where they ran off, and taken to be checked by army vets.
Four people were taken to hospital from three incidents: on Buckingham Palace Road, Belgrave Square, and the junction of Chancery Lane and Fleet Street, said London Ambulance Service.
It is understood three Household Cavalry personnel were assessed in hospital but that their injuries were not serious.
Photographer Jordan Pettitt witnessed the horses running down Aldwych, with the white horse’s front “saturated rich red”.
He told Sky News the atmosphere was “eerie” as the street had fallen “completely silent” despite it being rush hour.
Mr Pettitt added: “It was a very surreal moment, it’s not something I would ever expect to see in London nor would anyone expect to see in London.”
A management consultant, named only as Tula, said the sight was “gruesome” and “like a weird dream”.
“People were stopping in the street shocked,” they said.
“The horses were running into fast-moving traffic and seemed terrified. Some unmarked police cars were chasing after them, which didn’t seem to be helping.”
‘In a frenzy’
London Ambulance Service said paramedics were first called to Buckingham Palace Road to reports of a person being thrown from a horse.
A taxi driver waiting at the nearby Clermont Hotel had the windows of his Mercedes people carrier smashed by one of the horses.
Another of the animals then collided with a parked double-decker tour bus, smashing the windscreen.