After Trooping the Colour, members of the royal family appeared on the balcony of Buckingham Palace to watch a military flypast, including Prince William and Kate with their children George and Charlotte.
Prince George, who is nearly three, waved at the planes as they flew by, while one-year-old Princess Charlotte, who was carried by Kate, blocked her ears.
The parade featured almost 1,500 uniformed officers and their men as well as more than 300 horses.
But it was the queen’s unusually bold dress and hat combination in a green so bright as to be almost fluorescent which caught the eye of many. On Twitter, some compared it to the colour of Kermit the Frog from Jim Henson’s TV comedy “The Muppet Show”.
The queen favours bright colours for public appearances so she can be easily seen by crowds, though they are normally in more muted shades.
On Saturday, she was accompanied by her husband Prince Philip, who wore a ceremonial red military uniform and bearskin hat. Thousands of people flocked to see the spectacle.
The queen celebrates two birthdays as part of a royal tradition which dates back over 250 years. The actual date of her birth is April 21, 1926. But her official birthday is also marked in Britain on a Saturday in June with Trooping the Colour.
The birthday parade features the Household Division of the British Army — seven regiments traditionally seen as the monarch’s official guards.
On Sunday, a giant street party known as the Patron’s Lunch will be held on The Mall outside Buckingham Palace to mark the queen’s official birthday. The celebrations started on Friday with a service of thanksgiving at St Paul’s Cathedral.
Even Russian President Vladimir Putin, whose country has frosty relations with Britain, sent a message of congratulations to the queen.
The Kremlin said Putin “noted the rich history of relations between the two countries and expressed confidence in the further development of constructive cooperation”.