Thousands of people turned out to watch the annual Trooping the Colour parade on the Queen's official 90th birthday.
Dressed in a vivid lime green coat and matching hat, the Queen was escorted down the Mall in a horse-drawn carriage to the ceremony at Horse Guards Parade.
After the parade of more than 1,600 soldiers and 300 horses, she appeared on the balcony of Buckingham Palace with members of her family, including Princess Charlotte, for an RAF flypast.
Gun salutes were fired across the UK.
In the afternoon, the royal barge Gloriana led a flotilla of about 50 boats in a pageant down the River Thames, carrying figures including five-time Olympic gold rower Sir Steve Redgrave.
Visible in her vibrant ensemble, the Queen smiled and waved to crowds as she was driven with the Duke of Edinburgh in an open-top carriage up the flag-lined Mall.
She has attended the annual event - also known as the Queen's birthday parade - every year of her reign, except in 1955 when it was cancelled due to a rail strike.
The Duchesses of Cornwall and Cambridge, both dressed in white, travelled together in another carriage with Prince Harry.
The Duke of York and his daughters, Princesses Beatrice and Eugenie, in characteristically striking headwear, followed behind.
The Prince of Wales, who is Colonel of the Welsh Guards, the Princess Royal, Colonel of the Blues and Royals, and the Duke of Cambridge, Colonel of the Irish Guards, rode on horseback in their ceremonial uniforms.
The Queen herself rode on horseback in the parade until she was in her 70s, riding side-saddle and wearing the uniform of the regiment whose Colour - or flag - was being trooped.
This year the Colour being trooped belonged to 7 Company Coldstream Guards.
Spectators in the packed stands rose to their feet as a mark of respect as the Queen's carriage drew into the parade ground - Henry VIII's former jousting yard.
Among them were Prime Minister David Cameron and his wife Samantha, as well as many families of the servicemen and women taking part.
Weeks of preparation go into the tightly choreographed spectacle, which includes mounted military bands and Guardsmen wearing traditional bearskin hats and scarlet tunics.
The massed bands of the foot guards performed music, including an arrangement of Happy Birthday, and an intricate display of marching manoeuvres before the Queen received the royal salute.
The Royal Family then gathered on the balcony of Buckingham Palace to watch an RAF flypast of aircraft including two Spitfires, four helicopters and the Red Arrows.
Princess Charlotte, who joined her family on the balcony for the first time, covered her ears as two Tornados flew overhead.
The gathered crowds gave three cheers as the ceremony came to an end.
Later in the afternoon, the million-pound royal barge Gloriana was rowed down the Thames by crew of both able-bodied and adaptive rowers with disabilities.
It led a flotilla that included boats from every decade the Queen has been monarch.
Oars were raised on Gloriana as Tower Bridge was opened for the event.
Throughout the weekend a number of street parties will be taking place around the UK in honour of the Queen's official birthday.
She celebrates two birthdays every year - her actual birthday on 21 April, and her official birthday held on a Saturday in June - in a tradition going back 250 yearsto try to ensure better weather for the monarch's official celebrations.
Archbishop of Canterbury Justin Welby said the Queen had reigned through "war and hardship, turmoil and change".
'A symbolic affair'
By Peter Hunt, BBC Royal correspondent
This was an unmissable monarch - wearing, as she was, a vibrant, lime green coat - at an annual occasion she's never missed - an occasion where the dominant colour is usually the scarlet of the ceremonial soldiers' tunics.
The guardsmen performing for their colonel in chief on the parade ground were fighting soldiers.
Centuries ago, the colour or flag was a rallying point on the battlefield.
It's now a symbolic affair. Today, it was held in honour of the Queen who inspected her troops - as Queen Victoria did once, and as all monarchs have done since the time of Edward VII.
The annual pageant ended with the fly past and the Queen and other royals appearing on the balcony.
Dedicated royal watchers will have been delighted by the sight of Princess Charlotte and an animated Prince George.
With Prince Charles and Prince William also there, this was the British monarchy, its present and its future on display.