Celebrations are taking place around the UK on the day the Queen turns 90.
Crowds of cheering people lined the streets in Windsor as the monarch took part in a walkabout, and royal gun salutes have been fired from each of the UK's capital cities.
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.
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."
The Queen, who was accompanied by the Duke of Edinburgh during her tour in Windsor, 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.
Later on Thursday, the Queen will light symbolic beacons in Windsor.
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.
The crowds were 10 deep in places, with locals from Windsor desperate to see their royal neighbour standing alongside those from Canada, America and Japan. They came wearing Union Jack ties and wigs, and tights embellished with crowns.
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 were due to be 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 reading by Prince Charles, which has been broadcast by the BBC, is an extract from a speech by Archbishop Thomas Cranmer to King Henry VIII after the birth of the future Queen Elizabeth I.
Earlier, a photograph was 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.
In the evening the Queen will be accompanied by Prince Philip, the Prince of Wales, and the Duchess of Cornwall, as she lights the first of more than 900 beacons across the UK and the world to mark her birthday.
Members of the Army cadet force will take beacons to the top of the highest peaks of England, Scotland, Wales and Northern Ireland.
Some of the beacons will be specially-built gas-fuelled structures, while others will be traditional bonfires or braziers on top of tall wooden posts.
To coincide with the Queen's birthday, the largest exhibition of the Queen's clothes and accessories ever shown in Scotland will open at the Palace of Holyroodhouse later.
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.