British Prime Minister David Cameron led the tributes to Queen Elizabeth II on Thursday, hailing her as a "rock of strength," as Britain's longest-serving monarch turned 90.

"No other country has a head of state with such wisdom and confidence," Cameron said in a formal tribute in parliament.

The queen carries herself with "extraordinary grace and humility" and had attended 177 public engagements last year alone, he said.

Opposition Labour Party leader Jeremy Corbyn backed Cameron's tribute and praised the queen for fulfilling her duties with "enormous warmth."

In an earlier video message on YouTube, Cameron said the queen had "devoted her entire life to the service of others, representing our country."

"Rarely has anyone in public life served for so long, served so brilliantly, worked so hard, and brought so many people together," he said.

"And in this modern Elizabethan era, in which so much around her has changed, Her Majesty has been steadfast, a rock of strength for our nation, for our Commonwealth, and on so many occasions, for the whole world," Cameron said.

The queen, who has ruled Britain for 64 years, was scheduled to attend several public events, culminating in her lighting the first of hundreds of beacons on Thursday evening to mark her birthday across Britain.

Thousands of people gathered in the town of Windsor, where the queen was scheduled to leave nearby Windsor Castle, her royal residence outside London, to meet members of the public during what her office called a "Windsor walkabout" on Thursday afternoon.

She will reportedly eat lunch at Windsor Guildhall with a group of fellow 90-year-olds and receive a birthday cake made by Nadiya Hussein, a British Muslim woman who won last year's final of The Great British Bake-off, a BBC reality show.

Prince Charles, 67, her eldest son and heir to the throne, issued an audio message via the BBC on Thursday, quoting an edited passage from Shakespeare's Henry VIII.

"She shall be, to the happiness of England, an aged princess; many days shall see her, and yet no day without a deed to crown it," Charles said.

Prince William, Charles's son and second in line to the throne, on Wednesday praised the "strong female influence" of his grandmother following the death of his mother, Princess Diana, in 1997.

The royal family also published three anniversary portraits of Queen Elizabeth II by celebrity photographer Annie Leibovitz on Thursday.

Following Thursday's celebration of her actual birthday, more events are planned in June to mark the queen's official birthday, including a service at London's St Paul's Cathedral and a street party for thousands of people.

Born in 1926, the queen has ruled since 1952, when she was 25 years old. In September, she surpassed the six-decade reign of Queen Victoria, her great-great grandmother, from 1837 to 1901.

Some royal watchers said Queen Elizabeth's enduring popularity comes partly from her success in avoiding public controversy.

Britain's system of constitutional monarchy keeps the queen as a largely ceremonial head of state, allowing the government to pass legislation and run the country.

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