The 86-year-old monarch and her husband, Prince Philip, visited Epsom racecourse south of the capital for the Derby, one of the year's biggest horse-racing meetings. The queen waved to the 130,000-strong crowd as she was driven down the racecourse in a Bentley bearing the Royal Standard - the car's sun roof kept shut under gray skies - before settling down to watch the races from the royal box.
Dressed in a royal blue coat and matching hat over a blue-and-white floral dress, the queen was accompanied by members of the royal family including her sons Prince Andrew and Prince Edward and Andrew's daughters Princess Beatrice and Princess Eugenie.
The royals were treated to an aerial display by members of the British army's Red Devils parachute team before the main event - the racing.
The monarch is a racing fan and horse breeder who has attended the derby for decades and reads the Racing Post each day over breakfast, although unlike many of her subjects she does not gamble.
The queen presented prizes to some of the race winners and spoke intently to jockeys and trainers
"She's incredibly knowledgeable. Her knowledge of thoroughbreds and breeding goes way back," said Anthony Cane, chairman of Epsom Downs Racecourse.
The queen took the throne in 1952 on the death of her father, King George VI, and most Britons have known no other monarch.
Despite cool, damp weather in much of the country, hundreds of thousands of people are expected to participate in celebrations, including street parties, Sunday's 1,000-boat flotilla down the Thames, and a Monday pop concert in front of Buckingham Palace featuring Elton John and Paul McCartney.
Jubilee events end Tuesday with a religious service at St. Paul's Cathedral, a carriage procession through the streets of London and the queen's appearance with her children, grandchildren, and great-grandchildren on the palace balcony.