Queen Elizabeth II lit on Thursday 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 realized 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 recognize 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.
Most of the gun salutes were 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 Honorable 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, 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.
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. (BBC)