Britain's Queen Elizabeth II greets wellwishers during a 'walkabout' on her 90th birthday in Windsor, west of London, on April 21, 2016. (Photo: AFP)
Britain celebrated the 90th birthday of Queen Elizabeth II on Thursday with tributes to a popular monarch who has steered it through the decline of empire and a wave of scandals to the Internet age.
The sovereign, who has been on the throne since 1953, emerged from Windsor Castle dressed in a lime green outfit and matching hat to accept presents and flowers from hundreds of well-wishers.
Prime Minister David Cameron hailed her as "a rock of strength for our nation" while her son and heir Prince Charles marked the occasion by reading Shakespeare in a special broadcast on BBC radio.
Later on, the monarch, who last year overtook her great-great grandmother queen Victoria to become Britain's longest-reigning monarch, will host a family dinner to celebrate.
She will also light a beacon, the first in a chain of a thousand which will be lit around the country and the world, symbolising the length of her life and her reign.
In an address to the House of Commons to mark the occasion, Cameron said she had been "steadfast, a rock of strength for our nation, for our Commonwealth and on many occasions for the whole world".
He said Britain was "uniquely blessed" to have her and praised the queen's "unshakeable sense of duty" while noting she was now "starting to take things a little easier".
While still an active monarch, the queen has scaled back her duties in recent years as Prince Charles and grandson Prince William plus wife Kate take a more prominent role.
The royals remain popular but some analysts question what will happen when the queen -- who is in good health, enjoying regular rides on her favourite pony and walking her corgis -- reaches the end of her life.
In an editorial, The Times newspaper praised her as "a symbol of continuity and the best embodiment we have of a complex national identity".
But it warned the monarchy "will not long retain its popularity" if Charles, who often writes to ministers about issues close to his heart like the environment, is seen to breach constitutional convention by interfering in politics as king.
To mark the queen's birthday, there were two military gun salutes at London's Hyde Park and the Tower of London, and parliament will be lit up in the red, white and blue of the British flag.
Buckingham Palace also released new official pictures of the queen taken by US celebrity photographer Annie Leibovitz.
They were shot at Windsor Castle, where she will on Friday host US President Barack Obama and First Lady Michelle Obama.
In one photograph, the monarch is pictured with four of her beloved dogs outside the mediaeval castle west of London, and in another she is surrounded by some of the youngest members of her family, including a great-granddaughter clutching one of the queen's trademark handbags.
Prince Charles read an extract from William Shakespeare's "Henry VIII" about the future Elizabeth I in tribute to his mother on the BBC World Service.
"She shall be, to the happiness of England/ An aged princess," read the text, which also described her as "a pattern to all princes living with her/ And all that shall succeed".
In recent years, Britain's royal family has not always been on as sure a footing as it seems to be now.
It went through what the queen described as an "annus horribilis" or horrible year in 1992 when Prince Charles separated from his first wife Diana, while two of the queen's other children also went through breakups and Windsor Castle was hit by fire.
The monarchy's popularity plunged after Diana died in a Paris car crash in 1997 and the queen was initially accused of reacting coldly.
Advisors subsequently steered the royals towards a more modern image.
Some 68 percent of Britons think the monarchy is good for Britain, according to a YouGov survey last year.
Royal fan John Loughrey, who has slept on a bench outside Windsor Castle since Monday to be there for the queen's big day, described her as "unique".
"She hasn't put a foot wrong," said Loughrey, dressed in red, white and blue, before launching into a rendition of the national anthem.