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One wishes to celebrate one's 90th birthday

Britain's Queen Elizabeth II began her 90th birthday on Thursday surrounded by family, but she then marked the occasion by taking a leisurely stroll around the quiet town west of London that she often calls home.

It wasn't to be a quiet walk. Thousands of well-wishers had taken up spots early in the morning along Windsor's streets to wish the United Kingdom's longest-reigning monarch a happy birthday.

Prime Minister David Cameron released a statement early Thursday lauding the queen as an unflappable figurehead for Britons through the best and worst of the last six decades.

"Her Majesty has been steadfast -- a rock of strength for our nation, for our Commonwealth and on many occasions for the whole world," Cameron said, noting that she "has lived through some extraordinary times."

Buckingham Palace released three official portraits of the queen and her close family members Thursday to mark the occasion.

One of the images, captured by renowned photographer Annie Leibovitz, shows the queen surrounded by seven young grandchildren and great-grandchildren, including 11-month-old Princess Charlotte, whom she holds in her arms.

Elizabeth II becomes Britain's longest-reigning monarch

In another, the queen is pictured walking through the private grounds of Windsor Castle with four of her dogs. The castle built in the 11th century by William the Conqueror is one of the monarch's official residences.

At about midday on Thursday Queen Elizabeth II got out of a Range Rover wearing a pale green overcoat and matching hat to walk through Windsor and greet well-wishers.

After half an hour shaking hands and enjoying countless renditions of "Happy Birthday" by school groups and others along the procession route near the palace, the queen climbed back into the open-top Range Rover and headed, waving all the time, deeper into Windsor.

Gun salutes will reverberate from three locations on Thursday; Hyde Park in London, Windsor Great Park, and at the Tower of London, where the Crown Jewels are kept, to mark the royal birthday.

Thursday, April 21, is the Queen's actual birthday, but there will be another celebration to mark the British monarch's "official" birthday in June -- a date chosen, in part, to take advantage of better summer weather.