The Prince of Wales has paid a touching tribute to his mother, wishing her the "most special and happiest of birthdays".
Charles, at a beacon lighting ceremony at Windsor Castle to mark the Queen's 90th birthday, spoke of the "love and affection" for her throughout the country and the Commonwealth.
He called the head of state "Mummy" during his short address - often his affectionate introduction for the sovereign during royal celebrations.
As dusk approached on her milestone birthday, he said: "Your Majesty, Mummy, I find it very hard to believe you've reached your 90th year and I suddenly realised the other day that I've known you since you were 22 years old.
"This is, ladies and gentlemen, a very special occasion and the beacon Her Majesty is about to light will also represent, as it lights other beacons across the nation, the love and affection in which you are held throughout this country and the Commonwealth.
"So, ladies and gentlemen, can we wish Her Majesty the most special and happiest of birthdays on this occasion. And long may you reign over us.
"Now, ladies and gentlemen, can I ask you as well if you could raise three very special cheers for Her Majesty on this special occasion.
"And if they're loud enough it might just work to light the other beacons by spontaneous combustion."
The crowd then gave three cheers.
The Queen and the Duke of Edinburgh were driven to Cambridge Gate in Home Park, Windsor, for the lighting of the first in a network of more than 1,260 beacons.
She was dressed in a white silk headscarf for her last public engagement of the day before a private black tie dinner hosted by Charles in the Waterloo Chamber of the castle.
More than 30 royals were among more than 70 guests joining the Queen for the lavish affair.
Charles handed the torch to the Queen and invited her to light the principal beacon - a large brazier on a 20ft (6m) pole at the start of the Long Walk.
With the beacon towering above them Charles handed a flaming torch to the Queen who lit a fuse which set off a series of small controlled explosions which ignited the beacon.
It took a while to get going and Bruno Peek, who has organised a number of national beacon lighting events to mark royal anniversaries, said Philip quipped: "It's not going up".
The Royal party left for the Queen's private birthday dinner and expected among the guests were the Duke and Duchess of Cambridge, Prince Harry and other senior royals.
The Queen's day began with the monarch acknowledging the well-wishes of the thousands who filled Windsor town centre when she went on a walkabout.
And the birthday Queen declared 1926 - the year she was born - a vintage year when she met a group of fellow nonagenarians.
During the public meet and greet, with the Duke of Edinburgh by her side, the Queen received armfuls of cards, bouquets and even a cake as she acknowledged the greetings of the crowds.
The royal fans who lined crash barriers around the town centre broke into spontaneous renditions of Happy Birthday and applause as they caught sight of the monarch.
There were a few famous faces among the crowds, from the Gogglebox stars Sandra and Sandy, to reality TV celebrity Joey Essex and television chef Mary Berry.
The event was part of an unveiling ceremony at the foot of Castle Hill in Windsor that formally marked the launch of the Queen's Walkway .
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.
Historian Hugo Vickers, the Outdoor Trust's chairman, helped the Queen remove the cloth from a large temporary display board in the middle of a main road and she remarked it was a "lovely day".
Mr Vickers said: "She was terribly happy with the beautiful weather and the lovely occasion. She said it was a lovely day."
He added: "She loved the panel. Prince Philip said it wouldn't last long if we left it there."
At one point the Queen stopped with lady-in-waiting Jennifer Gordon-Lennox to briefly chat to sisters Judy and Anne Daley, from Cardiff, who were each holding a balloon that formed the number 90. The siblings had been featured on breakfast television earlier and in the excitement the number nine balloon had floated off over Windsor, but luckily another was donated by a local shop.
Anne, 55, said: "When the lady-in-waiting saw the balloons she said 'you've got the nine back'. The Queen was killing herself laughing, she must have seen it on TV, she was really lovely."
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.''
And when the Queen met a group of fellow nonagenarians - all born in 1926 - Rosamund Fulawaka, whose 90th birthday was on April 10, said she told them: "You were all born in a lovely vintage year."
Mrs Fulawaka, from Ascot, added: "That was a lovely thing to say.
"It was wonderful to meet her, I had seen her from a distance but never seen her close up so I had a really good look and thought she was just lovely."
Yetta Jacobs, who celebrated her 90th birthday on February 11, was evacuated to Windsor in 1939 from London's East End.
"I think she just said 'hello', she recalled after their meeting.
She added: "I told her I was evacuated to Windsor and she asked if I liked it and I said yes"
Gogglebox stars Sandra and Sandy had staked a spot in the crowd to get a good view of the Queen.
Sandra Martin, who was wearing a pair of union flag decorated glasses, said: ''We've taken time out of filming to come here and stayed overnight in Windsor.
''We're here to celebrate the Queen's birthday and I'm the only one on the programme who is allowed to speak about the Queen - for the last 20 years I've been a loyal, devoted follower.''
Philip stopped to talk to the pair and Ms Martin said later: "He asked me what I do and where I live and I told him I'm Sandra, I live in Brixton and I appear on TV.
"I told him I sit on my sofa and watch TV and you watch me watching TV and that made him laugh."
Town crier Tony Appleton, 80, from Chelmsford, had been at the Lindo Wing when Prince George and Princess Charlotte was born.
Prime Minister David Cameron has led the tributes to the Queen - born on April 21 1926 - who has become the country's first nonagenarian sovereign.
The Prince of Wales and the Duchess of Cornwall were married at Windsor's Guildhall and in its open air portico known as the corn market the Queen and Philip chatted to a group, mostly women, who turn 90 this year.
Nadiya Hussain, winner of the BBC's The Great British Bake Off, had made the Queen an orange drizzle birthday cake over four days, and used 42 eggs to create the three tier confection that actually featured 12 cakes.
She said: "My theory was if lemon drizzle was good enough for Mary Berry, we can change it up and do one for the Queen."
The baker added that she froze when she first spoke to the Queen but recovered: "She asked me about what was in the cake, because she said 'does it cut?' so clearly she's had issues about cutting cake - hence why I didn't do a fruit cake."
She added: "You've got to feel some pressure when it's the Queen right? Every time I tried to ignore the fact I was doing it for the Queen, my husband would very conveniently remind me - 'hey, hey you're slacking. It's for the Queen get up, you can't be lying down'."
And it appears the Duke may be an avid watcher of the BBC's hit baking show, Nadiya suggested. "The Queen said 'Oh she won the baking show' and he said 'of course I know she won the baking show' - I think they were having a bit of a conversation.