A 17-year-old man has been arrested for shooting a replica gun at the Queen as she rode past crowds on horseback.
He fired six blank cartridges before being overcome by a Guardsman and police.
The shots, which came just before 1100BST, startled the Queen’s horse, but she was able to bring it back under control within a few seconds.
The Queen had left Buckingham Palace 15 minutes earlier.
She had rode down the Mall and was turning into a crowded Horseguards’ Parade when the incident occurred.
The monarch looked shaken by the episode, but soon recovered her composure.
She comforted her 19-year-old horse, Burmese, which she has ridden in birthday parades since 1969.
The procession continued as planned, and afterwards the Queen returned to Buckingham Palace by the same route, under the close watch of security services.
This is not the first time a member of the royal family has faced danger from within a crowd of spectators.
Seven years ago, a few yards down the Mall, Princess Anne was attacked by a gunman.
Half a mile away in 1936 King Edward VIII faced a man with a loaded revolver.
And Queen Victoria was also shot at by a man with a gun in the Mall.
Security will take on an increasing importance as next month’s wedding between Prince Charles and Diana Spencer approaches.