Four days of celebrations for the Queen’s Platinum Jubilee kicked off Thursday morning, as a military parade marked the 70-year reign of Britain’s longest serving monarch, and tens of thousands of well-wishers thronged closed-down streets across central London.

It’s an event that is unprecedented in British history – no other monarch has made it to seven decades on the throne. But it also represents a moment of reflection for how Britain has changed since 1952, when the Queen inherited the job after the sudden death of her father. She was just 25 years old, a young mother of two.

“Thank you to everyone who has been involved in convening communities, families, neighbours and friends to mark my Platinum Jubilee, in the United Kingdom and across the Commonwealth,” the Queen said in a statement as the festivities got under way.

“I continue to be inspired by the goodwill shown to me.”

Thursday’s celebrations began with the Trooping of the Colour, a military procession that traditionally marks the monarch’s “official” birthday; the “Colour” is a regimental flag that was used as a rallying point in battle.

After proceeding along the Mall in central London to Buckingham Palace, the Royal Family appeared on the palace balcony to watch a royal flyover, with military jets spelling out the number “70″ and displaying the colours of the Union Jack in smoke trails.

In Trafalgar Square, which was packed with pedestrians picnicking outside Canada House and climbing the lion statues that surround Nelson’s Column, cheers echoed as the procession of helicopters and jets flew overhead.

Barbara Davies, 85, had gathered her extended family in the square on picnic blankets and folding chairs, a Union Jack draped over her shoulders. She estimated she had seen the Queen in person at least a dozen times, including on her Coronation Day in London, when Ms. Davies was a schoolgirl.

Asked what she remembered about the event, she recalled, “just how young [the Queen] was.” It was a bittersweet moment – the crowd was very aware that the Queen was being crowned so young due to the death of her beloved father.

Ms. Davies recalled the deep loyalty at the time to the Queen’s parents, especially after their decision to stay in London through the Blitz, and the heavy expectations that had been placed on the young Monarch’s shoulders – expectations that Ms. Davies said she has exceeded. “She’s been wonderful,” Ms. Davies said.

It was a sentiment of respect that was repeated across the international crowd, which cut across age and nationality.

The Jubilee has tried to walk the line of embracing how Britain has diversified and changed, while also leaning into the historical pomp of a royal event. Alongside a traditional church service on Friday and a horse race on Saturday, the holiday will also feature a concert with performances from Ed Sheeran and Diana Ross. Private street parties and events across the U.K. range from afternoon teas to Drag Queen brunches and a Bollywood-style gala.

But there’s no doubt the Royal Family is changing, too. The Queen, at 96, has been struggling with mobility problems, and her appearances at official events have started to slow. Last year, she lost her husband of 73 years, Prince Philip. Her heir, Prince Charles, has taken on a more active role, prompting questions about whether another monarch will be embraced as widely.

“It’s unprecedented for there to be a 70-year jubilee,” said Philip Murphy, director of the Institute of Commonwealth Studies at the University of London. “That raises a real question about what’s next, and whether this” – a constitutional monarchy – ”is actually quite a stable and popular form of government, or whether it’s really come down to the personality of the Queen.”

Other signs of discord were visible on Buckingham Palace’s balcony, which was reserved for “working” members of the Royal Family. While Prince Harry, his wife Meghan, Duchess of Sussex, and children are visiting from the U.S., they will not make an official appearance after leaving their roles in 2020.

Prince Andrew was also absent, after being stripped of his royal duties in January over allegations that he had sexually assaulted an underaged girl.

Andrew, the Queen’s second son, was forced to quit royal duties over his friendship with late convicted sex offender Jeffrey Epstein, and settled a U.S. lawsuit in February in which he had been accused of sexually abusing a woman when she was underage. Andrew, 62, officially known as the Duke of York, denied any wrongdoing.

Andrew will miss a Thanksgiving service being held for the Queen on Friday to mark her 70 years on the throne, after he tested positive for COVID-19.

“The duke has tested positive for COVID and with regret will no longer be attending tomorrow’s service,” a Buckingham Palace spokesman said.

The Queen also will not be in attendance for the service at St. Paul’s Cathedral as originally planned, after experiencing some mobility discomfort when she appeared at Thursday’s military parade, Buckingham Palace said.

Prince Charles will be inheriting a role whose direct powers have in fact been cut back under the Queen, said Bob Morris, an honorary senior research associate at the Constitution Unit at University College London, and the editor of a book on the role of monarchies in modern democracies.

But if the Jubilee is fraught with symbolism, it’s also a great excuse for a party. After 70 years, including the last several years of Brexit and the pandemic, Mr. Morris said his own neighbourhood is planning a rollicking street party.

“We are celebrating the survival of the monarchy and the Queen, and we’re celebrating our own survival, too.”

- with files from Reuters