London - Prime Minister Theresa May was expected to lead the tributes to Queen Elizabeth II on Saturday, as Britain's longest-serving monarch turned 92.
Ahead of the celebrations, May praised the queen's "service, dedication [and] constancy" as head of the 53-nation, British-led Commonwealth since 1952.
The queen, who has ruled Britain for more than 65 years, is scheduled to attend The Queen's Birthday Party, a concert at London's Royal Albert Hall featuring singers Tom Jones, Kylie Minogue, Craig David, Sting and Shaggy, and South African band Ladysmith Black Mambazo.
Prince Harry is set to speak at the event in his new role as president of The Queen's Commonwealth Trust. The concert comes at the end of the week when Britain hosted 53 countries for a meeting of Commonwealth heads of government.
Celebration plans included soldiers from the Royal Horse Artillery parading their horses and gun carriages past Buckingham Palace, the queen's London home, and a 41-gun royal salute in nearby Hyde Park.
A second artillery company also planned to honour the queen with a 62-gun salute at the Tower of London.
Saturday's celebrations mark the actual date of the queen's birth, while more events are planned in June to mark her official birthday.
Born in 1926, the queen has ruled since 1952 when she was 25 years old.
In 2015, she surpassed the six-decade reign of Queen Victoria, her great-great grandmother, from 1837 to 1901.
Britain's system of constitutional monarchy keeps the queen as a largely ceremonial head of state, allowing the government to pass legislation and run the country.