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 "gentle constancy, royal dignity and kindly humanity".
Archbishop of Canterbury Justin Welby said the Queen had reigned through "war and hardship, turmoil and change".
The service was attended by many Royal Family members, including the Duke of Edinburgh - on his own 95th birthday.
At the service, which was attended by more than 2,000 people, Prime Minister David Cameron read a passage from the Bible.
And broadcaster Sir David Attenborough read a piece written by Michael Bond, author of the Paddington Bear books, on "the passing of the years". Both men also turned 90 this year.
It begins three days of events to mark the Queen's official birthday celebrations.
The Queen has two birthdays - her real birthday on 21 April, and her official birthday held on a Saturday in June - in a tradition going back 250 years. It was introduced to try to ensure better weather for the monarch's official celebrations.
After being temporarily delayed by traffic, the Queen arrived at St Paul's Cathedral dressed in yellow to cheers from the gathered crowds, and a fanfare of trumpets.
Guests included senior politicians, faith leaders and hundreds of members of the public nominated by government departments to recognise their service.
The Very Reverend David Ison opened the service by thanking the Queen 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."
At the service
BBC royal correspondent Peter Hunt
The Queen, born in the era of silent films, arrived at St Paul's Cathedral with the man who's been at her side for most of her adult life.
The Queen turned 90 in April; Prince Philip is 95 today.
The only acknowledgement of their age was the installation of a handrail to make it easier to get up and down the steps at the front of the cathedral.
The service, the church authorities said, had been designed to provide a balance between dignity and warmth.
It featured a new anthem - some music set to a German tune as a reminder of the Queen's ancestry - and members of the congregation who, like the Queen, have entered their tenth decade.
With Trooping the Colour on Saturday and a picnic on the Mall for 10,000 guests on Sunday, a long life and a long reign will continue to be celebrated.
To read more from Peter, click here.
'Wonder and gratitude'
Archbishop Welby, who led the service, read excerpts from Psalm 139 and told the congregation it explored "fear and wonder, and the connection between them".
He said over the Queen's lifetime, there had been "much to fear at times of personal challenge or national crisis".
"But just as the Psalmist sees through fear to something more stirring and more extraordinary, so we look back on Your Majesty's 90 years in the life of our nation with deep wonder and profound gratitude.
"Through war and hardship, through turmoil and change, we have been fearfully and wonderfully sustained."
More than 50 members of the Royal Family were present for the occasion, including the Duke and Duchess of Cornwall, the Duke and Duchess of Cambridge, and Prince Harry.
Among political figures were Labour leader Jeremy Corbyn, new Mayor of London Sadiq Khan and Scottish First Minister Nicola Sturgeon, as well as several cabinet ministers.
Broadcaster Clare Balding, who has an association with the Queen as three generations of her family have trained horses for her, was one of six individuals who led specially-written prayers.
'Shock of my life'
Another was read by Hilda Price, a 90-year-old widow from Cardiff who shares her birthday with the Queen.
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.
At a reception after the service at London's Guildhall, Prince Harry told guests the best birthday present the Queen could get would be a "day off so she could lie about and do nothing".
Meanwhile, the Queen headed back to Buckingham Palace to host a lunch for governors-general - who represent the Queen in Commonwealth countries.
The celebrations will continue on Saturday - which is the Queen's official birthday - with the annual Trooping the Colour 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.
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.
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