The Queen has lit the first of more than 900 beacons lighting up across the UK and overseas as part of her 90th birthday celebrations.
Prince Philip, the Prince of Wales and Duchess of Cornwall then joined the monarch at a private dinner in Windsor Castle.
Earlier, cheering crowds lined the streets in Windsor as the Queen took part in a walkabout.
On Twitter, the Queen thanked senders of “#HappyBirthdayYourMajesty” tweets.
In another tweet, the Queen said: “I send my best wishes to those who are celebrating their 90th birthday… on this shared occasion, I send my warm congratulations to you.”
At the lighting of the beacon in Windsor, the Prince of Wales, referring to his mother, said: “I find it very hard to believe that you’ve reached your 90th year and I suddenly realised the other day that I have known you since you were 22 years old.
“But this, ladies and gentleman, is a very special occasion and this beacon that her majesty is about to light will also represent – as it lights other beacons across the nation – the love and affection with which you are held throughout this country and the Commonwealth.
“So can we wish your majesty a special and the most happiest of birthdays on this occasion.”
Members of the Army Cadet Force have taken beacons to the top of the highest peaks of England, Scotland, Wales and Northern Ireland.
Some of the beacons are specially-built gas-fuelled structures, while others are traditional bonfires or braziers on top of tall wooden posts.
Earlier, royal gun salutes were fired from each of the UK’s capital cities as the Queen met crowds of well-wishers in Windsor.
The Queen, who was accompanied by the Duke of Edinburgh, was presented with a birthday cake at the Guildhall by the Great British Bake Off champion Nadiya Hussain, who had created an orange drizzle cake with a butter cream and marmalade filling.
The monarch also unveiled a plaque marking The Queen’s Walkway – a 6.3km trail that links 63 significant points in Windsor.
The trail was designed to recognise the moment on 9 September 2015 that the monarch broke the record held by her great-great-grandmother Queen Victoria, by being on the throne for 63 years and seven months.
This was, in many ways, an ordinary working day for the Queen. Unveiling a plaque, meeting flag-waving members of the public on a walkabout, being presented with a bouquet of flowers – these are things she has done thousands of times. But the crowds weren’t going to let her forget that this day, her 90th birthday, was something out of the ordinary.
Some had been waiting for hours – some for days – to catch a glimpse of the Queen on the day she became the nation’s first-ever nonagenarian monarch.
As the Queen and Duke of Edinburgh walked the short distance from Windsor Castle, a spontaneous rendition of Happy Birthday rang out – and it wouldn’t be the last.
One woman who has lived nearly as long as the Queen summed up the atmosphere. Gwen Tarr, 88, who says she is “Windsor born and bred”, said: “It was so lovely. I just wanted to celebrate the day with her and wish her a happy birthday – and many more birthdays to come.”