London: A beaming Queen Elizabeth II greeted flag-waving crowds outside St Paul`s Cathedral on Friday despite being delayed by London`s traffic as she made her way to celebrate her official 90th birthday.
The sprightly monarch who still keeps a gruelling calendar of official engagements and events kicked off three days of celebrations with a thanksgiving ceremony at the Baroque-style church.
The queen turned 90 on April 21, but British monarchs also have an official birthday celebration nearer the summer to make the most of the warmer weather in a tradition going back 250 years.
Dressed in canary yellow, the queen entered the 340-year-old cathedral to a fanfare sounded by state trumpeters of the Household Calvary, accompanied by husband Prince Philip, who himself celebrated his 95th birthday on Friday.
Senior royals including Prince Charles, Prince Harry, Prince William and his wife Kate were joined in the congregation by Prime Minister David Cameron, opposition Labour leader Jeremy Corbyn -- a noted republican -- and new London mayor Sadiq Khan.
Veteran naturalist and broadcaster David Attenborough, who was born in the same year as the queen, read "Reflections on the Passing of the Years" by Paddington Bear author Michael Bond.
Archbishop of Canterbury Justin Welby, paid tribute to the queen`s "wonderful service" during the sermon.
"You have been an instrument of God`s peace, and through you, God has so often turned fear into wonder -- and joy," he said.
There was a moment of drama when a member of the military fainted on the steps outside the cathedral while waiting for the service to arrive.
To mark the queen`s birthday, the royal family released a double portrait taken by celebrity photographer Annie Leibovitz of Queen Elizabeth II and the Duke of Edinburgh who have been married for more than 68 years.
The picture shows the queen in pale pink with Prince Philip wearing a shirt in the same colour.
The three-day celebrations have the theme of public service, with members of the NHS, the police and the fire service represented at the thanksgiving service.
The armed services will be honoured on Saturday -- the queen`s actual official birthday -- and Sunday will celebrate the work of charities.
The royal couple will host a lunch for visiting governors-general at Buckingham Palace following the service, while the streets of London will be lined with well-wishers on Saturday for the pomp of the Trooping of the Colour ceremony.
Around 10,000 guests are due on the Mall, the road leading to Buckingham Palace, for a street party on Sunday.