The Queen's official 90th birthday is being marked at the Trooping the Colour parade in central London.
More than 1,600 soldiers and 300 horses are taking part in the annual event on Horse Guards Parade.
Later the Queen will join other members of the Royal Family on the Buckingham Palace balcony for an RAF fly-past.
A flotilla of historic boats will also take part in a procession on the River Thames and royal gun salutes will be fired in several cities across the UK.
Dressed in a bright lime green coat and matching hat with pink flower, the Queen waved to crowds as she left Buckingham Palace in a horse-drawn carriage, accompanied by the Duke of Edinburgh.
She has attended the annual event - also known as the Queen's birthday parade - every year of her reign, except in 1955 when a national rail strike led to the event being cancelled.
Prince Harry, the Duchess of Cornwall and the Duchess of Cambridge travelled together in another carriage in the parade down the Mall.
The Duke of York and his daughters, Princesses Beatrice and Eugenie, followed in another carriage.
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.
Thousands of people have lined the Mall to watch the colourful spectacle, which includes mounted military bands and others wearing traditional bearskin hats.
Weeks of preparation go into the tightly choreographed event. This year the Colour - or flag - being trooped belongs to 1st Battalion Coldstream Guards.
The Massed Bands of the Household Division and the Mounted Band of the Household Cavalry played as the procession drew into Horse Guards Parade - Henry VIII's former jousting yard.
Street parties
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.
On Friday a national service of thanksgiving was held at St Paul's Cathedral paying tribute to the Queen's "faithful devotion" to the country.
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
Trooping the Colour is a military ceremony that dates back to the 18th Century.
The then-Princess Elizabeth first watched aged eight, from a balcony, when her grandfather George V was king.
As Queen she's taken part every year of her reign - and until the 80s rode on horseback - apart from in 1955 when it was cancelled because of a national rail strike.
Today it's a symbolic affair. Centuries ago, the colour or flag was a rallying point in battle, clearly visible above the smoke.
On a smoke-free Horse Guards Parade the Queen will once again take part. And then, once again, she will appear on the Buckingham Palace balcony with other royals - including those who represent the institution's future.
The 90-year-old monarch has a busy few days ahead. Prince Harry has suggested the best present for his grandmother would be a day off so she could put her feet up, lie about and do nothing.
Also on Friday, a host of celebrities and figures from public life were recognised in the Queen's Birthday Honours list.
Singer Rod Stewart received a knighthood while British astronaut Tim Peake becomes a Companion of the Order of St Michael and St George for services to space research and scientific education.
The final event of the three-day celebration will see the Queen host a street party for some 10,000 people on Sunday.
The Patron's Lunch will be 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 Queen spent her real 90th birthday on 21 April at Windsor Castle, where she lit the first of a chain of more than 900 beacons across the UK and overseas.