Windsor has been turned into a sea of union flags to welcome the Queen on her 90th birthday.
Die-hard royal fans, schoolchildren, well-wishers and tourists have flocked to the town centre in anticipation of the moment the Queen leaves her Windsor Castle home and goes on a walkabout.
Many of those who braved the chilly spring morning clutched presents for the Queen, from bouquets of flowers and home-made birthday cards to soft toys and balloons.
Fluttering above Windsor Castle's round tower was a huge version of the royal standard used for major celebrations.
It was hoisted over the tower when the Prince of Wales married the Duchess of Cornwall in Windsor's Guildhall in 2005.
David Cameron led the tributes to the Queen - born on April 21 1926 - who is the country's first nonagenarian sovereign.
The Prime Minister tweeted: "The whole country will want to wish the Queen a happy birthday today - she has been a rock of strength for our nation."
The milestone has been marked by the release of a historic photograph of the Queen surrounded by her five great-grandchildren - including Prince George and Princess Charlotte - and her two youngest grandchildren.
The image is one of three taken by celebrity photographer Annie Leibovitz, who has also captured the Queen informally with her daughter the Princess Royal, and in the final image with four of her dogs, corgis Willow and Holly and dorgis Vulcan and Candy.
One tribute will see the Houses of Parliament lit up in red, white and blue in honour of the Queen's milestone.
Traditional gun salutes will thunder across London when 41 volleys are fired by the King's Troop Royal Horse Artillery in Hyde Park, and 62 rounds by the Honourable Artillery Company close to the Tower of London - the extra 21 volleys for the citizens of the City of London to show their loyalty to the monarch.
Later the Queen, accompanied by the Duke of Edinburgh, will unveil a plaque marking the Queen's Walkway at the foot of Castle Hill in Windsor before going on a walkabout with Philip.
The walkway - a four-mile self-guided walking trail of Windsor by the Outdoor Trust - has been created in honour of the Queen becoming the country's longest reigning monarch.
In the evening the night sky over Windsor will be lit up by a beacon, started by the Queen, the first in a chain of more than 1,000 that will spread across the country and the globe.
After she lights the beacon, heir to the throne Charles will host a lavish private dinner in the castle for his mother, attended by her family and friends.
Lynn Lovell, 63, a retired teacher, said she made a spur-of-the-moment decision to join the crowds in Windsor after seeing the well-wishers on television.
She said: "This is a celebration, I live nearby in Langley, and it's almost as if it's a family event - we get to come along and be part of the party."
Ms Lovell, dressed head to toe in a union flag outfit, said: "She's phenomenal, she ageless and timeless. She's such a role model to all of us and her energy and enthusiasm for the work she does is an inspiration."
Nearby were sisters Judy and Anne Daley, from Cardiff, who were each holding a balloon that formed the number 90.
Judy, 50, a civilian police worker, said: "The Queen's just remarkable, a very formidable lady, and certainly the one we all look up to.
"She's remarkable for her age and her dedication to duty, day in and day out - just a wonderful, wonderful lady."