Buckingham Palace released this picture of the Queen, in her private audience room, to mark the day she becomes the longest reigning British monarch. Photograph: Mary McCartney/Crown copyright/ PA
British Prime Minister David Cameron led the tributes to the queen.
"Over the last 63 years, Her Majesty has been a rock of stability in a world of constant change and her selfless sense of service and duty has earned admiration not only in Britain, but right across the globe," Cameron said.
"It is only right that today we should celebrate her extraordinary record, as well as the grace and dignity with which she serves our country."
Those close to the queen say she is fairly blase about the milestone, believing it represents little more than the fact that she has lived for a long time.
"It's business as usual as far as she is concerned," said one senior aide.
Initially she did not even intend to mark the event publicly, but she has bowed to public pressure and will now undertake an official engagement in Scotland, where she traditionally spends her summer holiday.
Along with her husband Prince Philip, who has been at her side throughout her reign, she will take a journey on a steam locomotive to mark the opening of the longest domestic railway to be built in Britain for more than 100 years.
A Buckingham Palace spokeswoman said she might even make a rare public speech afterwards.
Background to the milestone
"It's inevitable that I should seem a rather remote figure to many of you, a successor to the kings and queens of history," she said in her first televised Christmas broadcast in 1957.
"I cannot lead you into battle. I do not give you laws or administer justice. But I can do something else. I can give you my heart and my devotion to these old islands and to all the peoples of our brotherhood of nations."
As a young princess, Elizabeth had not expected to become monarch as her father George VI only took the crown when his elder brother Edward VIII abdicated the throne in 1936 to marry American divorcee Wallis Simpson.
She was 25 when she ascended the throne on February 6, 1952, following her father's death.
That made her the 40th monarch in a royal line that traces its origin back to Norman King William the Conqueror who claimed the throne in 1066 with victory over Anglo-Saxon Harold II at the Battle of Hastings.
However, not everyone has been impressed, with republicans saying her silence on political matters is her finest achievement, and well-known British historian David Starkey remarking that she had never said or done anything memorable.
"She will not give her name to her age. Or, I suspect, to anything else," Starkey wrote in the Radio Times magazine.
Source: Al Jazeera and agencies