London Army and Emergency Services said four people were taken to hospital after spooked army horses broke loose during morning exercises in Central London and galloped through the streets at rush hour.
Photos and videos posted on social media by British news outlets showed two horses running along busy roads, dodging buses, taxis and other traffic.
Both were wearing saddles and bridles, with one apparently covered in blood on its chest and forelegs.
A number of prestigious Army cavalry regiments have stables in the British capital and horses are a regular sight around government buildings on Whitehall, Buckingham Palace, The Mall and in nearby Hyde Park.
London Ambulance Service said it was called at 8:25 am (0725 GMT) to reports of a person being thrown from a horse on Buckingham Palace Road.
No details were released on the injured people or the exact circumstances of their injuries.
The driver of a Mercedes car said he had been waiting outside a hotel in Buckingham Palace Road when he felt something smash into his car.
He said he saw three or four horses near the vehicle, and that an Army rider had been thrown off and injured.