A national service of thanksgiving to mark the Queen's 90th birthday at St Paul's Cathedral has paid tribute to her "faithful devotion" to the country.
Dean of St Paul's David Ison thanked the Queen for her "dutiful commitment, loving leadership, gentle constancy".
The service is being attended by Royal Family members, including the Duke of Edinburgh - on his own 95th birthday.
PM David Cameron read from the Bible, while broadcaster Sir David Attenborough will also give a reading.
The service is the beginning of three days of events to mark the Queen's official birthday celebrations - after she turned 90 in April.
The Queen has two birthdays - the official one on a Saturday in June, and her real birthday on 21 April - in a tradition going back 250 years. It was introduced to try to ensure better weather for the monarch's official celebrations.
Dressed in yellow, the Queen arrived at St Paul's Cathedral to cheers from the gathered crowds, and a fanfare of trumpets.
Archbishop of Canterbury Justin Welby is leading the service at St Paul's, while senior politicians, diplomats and governors-general - who represent the Queen in Commonwealth countries - are in attendance.
The Very Reverend David Ison said: "Most especially today do we give thanks for the length of years that has been granted to our most gracious Sovereign Lady, Queen Elizabeth: for her faithful devotion, dutiful commitment, loving leadership, gentle constancy, royal dignity and kindly humanity.
"And, as we give thanks for Her Majesty, so also do we give thanks for Philip, Duke of Edinburgh, and all the Royal Family: for mutual love and support and for service to this country and to the Commonwealth."
Broadcaster Clare Balding, who has an association with the Queen as three generations of her family have trained horses for her, will also be present.
Hilda Price, a 90-year-old widow from Cardiff who shares her birthday with the Queen, is one of six individuals leading specially-written prayers.
BEN STANSALL
BEN STANSALL
Great-grandmother Mrs Price, who was born on the same day as the Queen - 21 April 1926 - in a house in Carmarthen, said "it was the shock of my life" to be picked to take part in the service.
The service will begin a weekend of celebrations to mark the Queen's official birthday.
After the service, the Queen will host the governors-general for lunch at Buckingham Palace.
Analysis
PA
By Peter Hunt, BBC royal correspondent
Decades after their contemporaries embraced retirement, the Queen - who was 90 in April - and Prince Philip who turns 95 today - are still active in public life.
The prince will celebrate his birthday privately. According to the royal biographer, Hugo Vickers, the Queen's husband is still firing on all cylinders.
His wife will be the focus of three days of events being held to mark the passing of yet another milestone - Britain's longest reigning monarch is also Britain's oldest monarch.
At St Paul's this morning, the Queen will be joined by other nonagenarians including Sir David Attenborough and the author Michael Bond. Sir David will read a reflection on the passing of the years written by the creator of Paddington Bear.
The bear from deepest, darkest Peru has one thing in common with the head of state - they both have two birthdays. When Paddington went to live with the Brown family, they agreed bears had two birthdays every year "just like the Queen".
Street party
On Saturday - which is the Queen's official birthday - the annual Trooping the Colour will be held in London's Horse Guards Parade.
The birthday parade will end with members of the Royal Family making their annual appearance on the balcony of Buckingham Palace, where they will watch an RAF flypast.
ANNIE LEIBOVITZ
PA
FACUNDO ARRIZABALAGA
On Sunday, the Queen will then host a street party for some 10,000 people at the Patron's Lunch - a celebration of her patronage of more than 600 organisations in the UK and around the Commonwealth.
The Mall, in St James's Park, will be lined with picnic tables for the street party, during which guests will enjoy a hamper-style lunch.
The Duke of Edinburgh, Prince William and Prince Harry are due to attend the party.
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
