The event comes just days after her husband, Prince Philip, turned 90, with grandson Prince William taking part in the Trooping the Colour military processional.
Britain celebrates Queen Elizabeth II's 85th birthday
LONDON // Prince William and his new wife Catherine took part in the pomp and pageantry as Queen Elizabeth II celebrated her official birthday on Saturday.
William, her grandson, wore his wedding uniform – only this time complete with a bearskin hat for the traditional Trooping the Colour military parade in central London.
Dressed in the scarlet tunic of the honorary colonel of the Irish Guards, the Duke of Cambridge rode on horseback as Catherine joined other royals watching from a balcony.
It was the public's first chance to see the royal family all together since William and Kate's wedding on April 29, watched by an estimated two billion people around the world.
The event, celebrating the queen's 85th birthday, took place the day after her husband Prince Philip turned 90. He is the longest-serving consort in British history.
For the annual pageant, massed ranks of soldiers in the famous red tunics and bearskin hats of the British army march on Horse Guards Parade.
The Olympic beach volleyball competition will be held on the historic parade ground during the 2012 Games.
The pomp and ceremony of Trooping the Colour -- a hangover from preparations for battle when colours or flags were "trooped" down the rank so soldiers could recognise them -- marks the queen's official birthday.
Her actual birthday is on April 21 but historically the monarch has another in the summer months, because the weather is supposed to be better for open-air celebrations.
This year the colour paraded was that of the 1st Battalion Scots Guards.
There were huge cheers from the crowd as the royals left Buckingham Palace.
The queen and Prince Philip -- also wearing a bearskin -- rode in a phaeton carriage.
As regimental colonels, their eldest children, Prince Charles and Princess Anne rode alongside William, as did the queen's cousin Prince Edward, the Duke of Kent.
Catherine joined other royals including her brother-in-law Prince Harry in watching from a room that was once military hero the Duke of Wellington's office.
After the ceremony, the royals were to head back to Buckingham Palace to gather on the balcony for the first time since William's wedding.
This time Queen Elizabeth and Prince Philip will take centre stage as they watch the traditional flypast.
Twenty-five aircraft of 11 different types will take part, including a World War II Spitfire, Hurricane and Lancaster, modern Typhoon fighters and the Red Arrows aerobatics display team.
Prince Philip received a new royal title on his 90th birthday Friday, as the outspoken consort said he would finally scale back his workload.
The queen made him Lord High Admiral -- the titular head of the British Royal Navy -- partly in recognition of the promising seafaring career which Philip gave up to spend a lifetime at her side.
The plain-speaking patriarch spent his birthday with a characteristic lack of fuss, hosting a charity reception and chairing a conference for military colonels.