The nation marked the Queen's 86th birthday on Saturday with 41-gun salutes at Hyde Park and the Tower of London, while the monarch herself celebrated with family.
At Hyde Park in central London, there was a brief interruption to the King's Troop Royal Horse Artillery's salute when a panicked horse broke free in front of a crowd of nearly a thousand spectators, an AFP photographer saw.
An army spokeswoman said no one had been injured in the incident.
"The horse was having the time of his life," the spokeswoman told AFP. "He just wanted to join his mates. These are extremely well-trained horses, but you just never know when you're working with animals."
Gun salutes also took place at several military bases across the country to mark the occasion.
The queen's official birthday is celebrated each June, in the hope of warmer weather for the annual Trooping the Colour military ceremony.
This year's Trooping the Colour on June 16 will be part of a packed summer schedule for the queen, who is marking her diamond jubilee with a tour of Britain and is set to open the London Olympic Games on July 27.
Britain is celebrating the queen's 60 years on the throne with a four-day weekend in June that will include a pageant of boats on the River Thames and a concert outside Buckingham Palace featuring Paul McCartney and Elton John.
The queen appeared in good spirits on Friday as she chatted with world golf number one Rory McIlroy and his girlfriend, the top Danish tennis player Caroline Wozniacki, at the horse races in Newbury, west of London.
The monarch is a famous lover of horse racing, like her daughter Princess Anne who rode the queen's horse in the 1976 Montreal Olympic Games, and her grand-daughter Zara Phillips, a former world eventing champion.
Source: AFP European Edition