On her 90th birthday, Queen Elizabeth II tweets her thanks to well-wishers

reporting from London —  Queen Elizabeth II, Britain’s longest-reigning monarch, celebrated her 90th birthday on Thursday with a day full of pomp, ceremony and pageantry.

The monarch decided to spend her landmark birthday at her royal residence, Windsor Castle, where she spends the majority of her time.

Smiling broadly and with her 94-year-old husband Prince Philip by her side, she took part in a walkabout in the nearby town of Windsor, west of London, during which she was inundated by bouquets of flowers and children singing “Happy Birthday.”

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She was also driven through the streets in an open-top Range Rover to wave at the thousands of well-wishers who had gathered to celebrate her long reign.

Prime Minister David Cameron described her as a “rock of strength for our nation” during his personal tribute in the House of Commons.

The Prince of Wales also recorded a radio broadcast for his mother, reading from an edited script of Henry VIII by Shakespeare.

Good wishes also poured in from across the country and around the globe.

In a sign of how much the world has changed since 25-year-old Princess Elizabeth became queen, she sent out a thank-you message via Twitter to those posting messages using the hashtag #HappyBirthdayYourMajesty, which trended on social media.

She also acknowledged those that share her birthday.

“I send my best wishes to those who are celebrating their 90th birthday ... on this shared occasion, I send my warm congratulations to you,” the tweet read.

To mark the historic occasion, Buckingham Palace released a series of family portraits by celebrity photographer Annie Leibovitz.

In them, the queen is seen surrounded by the youngest members of the royal family, including the Duke and Duchess of Cambridge’s children, Princess Charlotte and Prince George, the future king.

In one image, Elizabeth is also photographed with her beloved Corgi dogs.

Elizabeth became queen in 1952, her rise to the throne only possible because of the abdication of her uncle, King Edward VIII, and the untimely death of her father, King George VI.

When she became queen, Harry Truman was president and Winston Churchill was prime minister. Josef Stalin also ruled the now-nonexistent Soviet Union.

She has reigned during the tenures of 12 prime ministers -- and 12 U.S. presidents -- and was on the throne as man first landed on the moon, the Cold War ended and peace was achieved in Northern Ireland.

She became Britain’s longest-serving monarch last September, surpassing the record held by her great-great-grandmother, Queen Victoria, but chose to keep the milestone a low-key affair out of respect for her ancestor’s memory.

Her birthday in the public eye began in the late afternoon when the queen greeted some of the royal fans who had camped outside Windsor Castle overnight to secure the best vantage spot for her walkabout.

Many were clad head to toe in the Union Jack and excitedly waved flags.

She also unveiled a plaque on the Queen’s Walkway in Windsor, which opened last year, and at noon gun salutes took place across Britain.

In the evening, she lit a symbolic beacon, the first of more than 900 to light up across Britain and around the world. Military cadets will take some beacons to the top of the highest peaks in England, Scotland, Wales and Northern Ireland.

The day also will be marked by leaders across the world, including by President Obama and Michelle Obama, who are scheduled to have lunch with the queen on Friday.

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The queen has two birthdays. April 21 is her actual date of birth but her official birthday takes place on a Saturday in June.

More official celebrations to mark her 90th birthday have been planned for the weekend of June 10-12, when the Mall, the sweeping road leading to Buckingham Palace, will be transformed into a huge street party, including a picnic area with blankets, hampers and Union Jack flags.

Boyle is a special correspondent.

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FOR THE RECORD

April 22, 4:12 p.m.: An earlier version of this article incorrectly identified Queen Elizabeth II’s father as King George IV. He was King George VI.

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