Media playback is unsupported on your device Media caption How the Queen spent her 90th birthday

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.

Image copyright PA Image caption The Queen has lit the first beacon of more than 900 that will light up across the UK and overseas as part of her 90th birthday celebrations

Image copyright PA Image caption Crowds gathered to watch the lighting of the first beacon

Earlier, royal gun salutes were fired from each of the UK's capital cities as the Queen met crowds of well-wishers in Windsor.

Media playback is unsupported on your device Media caption More than 900 beacons were set alight across the UK

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

Image copyright AP

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."

Well-wishers enjoy 'special day'

Cameron and Corbyn lead tributes

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.

Media playback is unsupported on your device Media caption Two ladies from Cardiff set off at 3am for the chance to see the Queen - Daniela Relph reports

Image caption The Queen and the Duke of Edinburgh travelled the streets of Windsor in an open-top Range Rover

Image copyright AFP/Getty Images Image caption The Queen was presented with a birthday cake at the Guildhall in Windsor by the Great British Bake Off champion Nadiya Hussain

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."

Image copyright PA Image caption The Queen was presented with flowers from the crowd in Windsor

Image copyright Getty Images Image caption People from Windsor, Canada, America and Japan cheered the Queen in Windsor

Image copyright Getty Images Image caption There was a gun salute at Edinburgh Castle

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.

Media playback is unsupported on your device Media caption Prince Charles pays Shakespearean tribute to the Queen

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.

Image copyright Annie Leibovitz/PA Image caption The Queen with her great-grandchildren and youngest grandchildren. From left: James, Viscount Severn; Lady Louise Windsor; Mia Tindall (holding the Queen's handbag); Princess Charlotte; Savannah Phillips; Prince George and Isla Phillips

Image copyright Annie Leibovitz/PA Image caption The Queen on steps at the rear of the East Terrace and East Garden with her four dogs, her corgis Willow and Holly and dorgis Vulcan and Candy

Image copyright Annie Leibovitz/PA Image caption One of the pictures is an informal shot of the Queen with her daughter, Princess Anne

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?

Image copyright Getty Images Image caption Monarchy fans were out early in Windsor, waiting for the Queen's walkabout

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

Full details of Queen's birthday celebrations

World media mark Queen's 90th birthday

In pictures: The Queen at 90 in 90 images

iWonder - Britain’s longest reigning monarch

Full coverage: The Queen at 90

Image copyright AFP/Getty Images Image caption Well-wishers gathered in Windsor ahead of the Queen's walkabout

Image caption BBC correspondent Christian Fraser spotted a London Underground tribute to the Queen at just the right moment

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.

Image copyright Reuters Image caption The largest exhibition of the Queen's clothes and accessories ever shown in Scotland will open at the Palace of Holyroodhouse

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

Find out more about Queen Elizabeth II on BBC iWonder