The Queen is celebrating her 91st birthday privately at Windsor Castle, as online tributes and military salutes are also being held in her honour.
Guns will fire at London's Hyde Park at 12:00 BST on Friday to mark the day.
The Queen, who reached her 65-year Sapphire Jubilee in February, is Britain's longest-reigning monarch.
This year it will be marked by traditional gun salutes and Buckingham Palace has tweeted a picture of the monarch as a baby.
Well-wishers used the hashtag #HappyBirthdayHerMajesty to tweet their own birthday messages.
The Royal Albert Hall said: "Happy birthday to our patron, Queen Elizabeth!" and a Scout group in Bristol tweeted: "#HappyBirthdayHerMajesty... And wish you a prosperous day!"
The Queen also celebrates an "official birthday" which falls on the second Saturday in June.
It will be marked with a parade in London known as the Trooping the Colour ceremony - this year's takes place on 10 June.
Two days earlier, the snap general election announced for Thursday 8 June will be the 17th of the Queen's reign.
The Queen typically spends her actual birthday privately, but the occasion is marked publicly by gun salutes in central London at midday.
There will be a 41-gun salute in Hyde Park, a 21-gun salute in Windsor Great Park and a 62-gun salute at the Tower of London.