An estimated 20,000 people cheered Queen Elizabeth II as she appeared outside Windsor Castle in a pink suit and hat on her 80th birthday.
A military band played as the crowd launched into an impromptu rendition of the traditional song "Happy Birthday to You." Many in the crowd presented the monarch with bouquets of flowers.
Meanwhile, a 41-gun salute echoed through in London's Hyde Park. A formal dinner for the royal family hosted by the queen's son, Prince Charles, at a palace in the city's Kew Gardens is capping off the evening.
Buckingham Palace says Queen Elizabeth has received 37,000 birthday cards and e-mails from around the world. She issued a statement saying she is very touched by the messages.
Princess Elizabeth became Queen Elizabeth in 1952, when her father, King George VI, died.
She is Europe's longest-serving living monarch.
Some information for this report was provided by AFP, AP and Reuters.