Elizabeth, 86, is the second British monarch -- after her great-great-grandmother, Queen Victoria -- to reign for at least 60 years.
Wearing a mint-green silk tulle ensemble, the queen participated in a service at St. Paul's Cathedral where trumpeters lined the stone steps and the Archbishop of Canterbury paid tribute to her "lifelong dedication," the BBC said. Prime Minister David Cameron gave a reading at the service.
After the religious ceremony, onlookers outside the church sang the national anthem as the queen headed to a reception at nearby Mansion House, the official residence of the Lord Mayor of London, followed by lunch at Westminster Hall, the BBC said.
She and other senior royals then returned by carriage through cheering crowds to Buckingham Palace ahead of a planned Royal Air Force flypast.
The queen's 90-year-old husband Prince Philip missed out on the events after he was hospitalized Monday for a bladder infection.