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.
At the scene
By Lauren Turner, BBC News reporter
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."
Most of the gun salutes consisted of 21 shots - the standard royal gun salute - at locations including Hillsborough Castle, Cardiff Castle, and Edinburgh Castle.
In London, the King's Troop Royal Horse Artillery staged a 41-gun salute at midday in Hyde Park. And the Honourable Artillery Company fired a 62-gun salute across the Thames from the Tower of London at 13:00 BST.
Prime Minister David Cameron said the Queen had been "a rock of strength for our nation" and the Commonwealth, as he and fellow politicians paid tribute in the House of Commons.
Mr Cameron said: "Her Majesty The Queen has lived through some extraordinary times in our world.
"From the Second World War... to the rations with which she bought the material for her wedding dress.
"From presenting the World Cup to England at Wembley in 1966, to man landing on the moon three years later. From the end of the Cold War to peace in Northern Ireland.
"Throughout it all, as the sands of culture shift and the tides of politics ebb and flow, Her Majesty has been steadfast - a rock of strength for our nation, for our Commonwealth and on many occasions for the whole world."
The Prince of Wales recorded a special radio broadcast for the day, in which he read an edited passage from William Shakespeare's Henry VIII.
The reading by Prince Charles, and broadcast on the BBC, was an extract from a speech by Archbishop Thomas Cranmer to King Henry VIII after the birth of the future Queen Elizabeth I.
A photograph was also released showing the monarch with young members of the Royal Family.
The image, one of three taken by celebrity photographer Annie Leibovitz, shows the Queen surrounded by her five great-grandchildren and her two youngest grandchildren.
The other Leibovitz photographs show the monarch walking in the grounds of Windsor Castle with four of her dogs and sitting with her daughter, the Princess Royal.
The image of the Queen with the young royals was taken in the Green Drawing Room, part of Windsor Castle's semi-State apartments just after Easter.
In it, the Queen - in the tradition of royal portraiture - holds her youngest great-grandchild Princess Charlotte, who is 11 months old, in her arms.
Also appearing in the image is two-year-old Prince George, Zara Phillips's two-year-old daughter Mia Tindall, who holds the Queen's famous black handbag, and Peter Phillips's daughters Savannah, five, and three-year-old Isla.
The Queen is also joined by the two youngest of her eight grandchildren - the Earl and Countess of Wessex's children - James, Viscount Severn, eight, and Lady Louise Windsor.
Queen's 90th: What's happening and when?
- 20 April - The Queen visited the Royal Mail delivery office and opened a bandstand in Windsor
- 21 April - She unveiled a plaque on Windsor's Queen's Walkway, before lighting the first of a chain of beacons. Gun salutes were held across the UK
- 12-15 May - Celebrations featuring hundreds of horses and performers in the grounds of Windsor Castle
- 10 June - A national service of thanksgiving at St Paul's Cathedral
- 11 June - The annual Trooping the Colour at Horse Guards Parade
- 12 June - The Queen will host a street party for 10,000 people in the Mall
To coincide with the Queen's birthday, the largest exhibition of the Queen's clothes and accessories ever shown in Scotland has opened at the Palace of Holyroodhouse.
The display has been selected to cover the Queen's life and reign, and includes evening and day wear.
On Friday, US President Barack Obama and First Lady Michelle Obama will be joining the Queen for lunch at Windsor Castle.
Every year the Queen celebrates two birthdays, with her actual birthday on 21 April and her official birthday on a Saturday in June.
Celebrations of her official birthday this year take place from 10-12 June.
Queen Elizabeth II at 90
- The Queen has been married for 68 years, longer than any other British monarch
- Her coronation was the first to be televised and was watched by over 20 million people in the UK
- There have been 12 UK prime ministers, seven Archbishops of Canterbury and seven Popes during her reign
- She has sat for more than 130 official portrait paintings
- During her reign, she has visited 117 countries, travelling more than a million miles
- In 2012 she became the second British monarch to celebrate a diamond jubilee, and in 2015 became the UK's longest reigning monarch