British Prime Minister David Cameron has led tributes to Queen Elizabeth II as she turns 90.
He reflected on her 64-year reign saying: "Her Majesty has devoted her entire life to the service of others".
"Representing our country and carrying out all her duties with extraordinary grace and humility".
"Rarely has anyone in public life served so long, served so brilliantly, worked so hard and brought so many people together".
"And in this modern Elizabethan era in which so much around her has changed, Her Majesty has been steadfast, a rock of strength for our nation, for our Commonwealth and on so many occasions, for the whole world".
PM: Rarely has anyone in public life served for so long, served so brilliantly. #HappyBirthdayYourMajesty #Queenat90https://t.co/pr6gIqr4wj
— UK Prime Minister (@Number10gov) April 21, 2016
She has met members of the public gathered outside Windsor Castle waving flags and singing to mark the occasion.
She was dressed in what was described by Buckingham Palace as a "spring grass green" wool and silk weave coat and dress with hat to match adorned with flowers by her dressmaker Angela Kelly for her birthday appearance.
Her arrival was marked by a rendition of 'Happy Birthday' by the Band of the Coldstream Guards.
She was accompanied by her husband of more than 68 years, Prince Philip, who himself turns 95 in June as she opened a walkway created in her honour.
The couple then went on to meet other nonagenarians, including Chelsea Pensioners, inside the town's Corn Market.
She was also presented with a cake by 'Great British Bake Off' winner Nadiya Hussain.
The Queen cuts her cake baked by Great British Bake off 2015 winner Nadiya Hussain #HappyBirthdayYourMajesty pic.twitter.com/8cEiFXJznG
— BritishMonarchy (@BritishMonarchy) April 21, 2016
They were then driven around the streets of Windsor in an open-topped Range Rover so they could wave at the cheering crowds.
Earlier, the King's Troop Royal Horse Artillery rode their horses and gun carriages past Buckingham Palace to Hyde Park to stage a 41 Gun Royal Salute at midday.
Prince Charles recorded a passage from William Shakespeare's Henry VIII to mark his mother's milestone.
He chose an edited extract from a speech by Archbishop Thomas Cranmer to King Henry VIII after the birth of the future Queen Elizabeth I.
He said: "She shall be, to the happiness of England, An aged princess; many days shall see her, And yet no day without a deed to crown it".
The Royal Navy has also put this tribute together:
#HappyBirthdayYourMajesty from the Royal Navy @BritishMonarchy #Queenat90https://t.co/WPoRyTm4gU
— Royal Navy (@RoyalNavy) April 21, 2016
And the British Army:
#HappyBirthdayYourMajesty. The Royal Tank Regiment wish Queen Elizabeth II a very happy birthday #Queenat90https://t.co/NZB6Hm4ipA
— British Army (@BritishArmy) April 21, 2016
In the evening, the night sky over her Berkshire home will be lit up by a beacon, started by Queen Elizabeth - the first in a chain of more than 1,000 that will spread across the country and the globe.
Her has been marked by the release of a historic photograph of her surrounded by her five great-grandchildren - including Prince George and Princess Charlotte - and her two youngest grandchildren.
The Queen surrounded by her two youngest grandchildren and her five great-grandchildren #Queenat90 pic.twitter.com/6iprDy0C3j
— BritishMonarchy (@BritishMonarchy) April 20, 2016
