Queen Elizabeth II on Thursday kicked off the first of four days of celebrations marking her record-breaking 70 years on the throne, to cheering crowds of tens of thousands of people.

Queen Elizabeth II will make two appearances on the balcony of Buckingham Palace on Thursday
Queen Elizabeth II will make two appearances on the balcony of Buckingham Palace on Thursday POOL via AFP / Andrew Matthews

But the 96-year-old sovereign's appearance at the Platinum Jubilee -- a milestone never previously reached by a British monarch -- took its toll, forcing her to pull out of a planned church service on Friday.

"The queen greatly enjoyed today's birthday parade and flypast but did experience some discomfort," Buckingham Palace said.

Crowds have been steadily building on The Mall outside the palace for several days
Crowds have been steadily building on The Mall outside the palace for several days AFP / Daniel LEAL

"Taking into account the journey and activity required to participate in tomorrow's national service of thanksgiving at St Paul's Cathedral, Her Majesty with great reluctance has concluded that she will not attend."

The extent of the queen's participation in the historic jubilee had already been in doubt due to her difficulties walking and standing, and a recent bout of Covid.

Elizabeth is the first British monarch in history to reign for 70 years on the throne and the celebrations have received blanket coverage
Elizabeth is the first British monarch in history to reign for 70 years on the throne and the celebrations have received blanket coverage AFP / Paul ELLIS

But dressed in dove blue, her hands clasped on a walking stick, she appeared on the Buckingham Palace balcony to take a salute after the centuries-old Trooping the Colour military parade.

Elizabeth has been queen since 1952
Elizabeth has been queen since 1952 AFP / David LORY

After gun salutes and a fly-past of military aircraft, she made a second appearance on the balcony with three generations of her heirs, princes Charles, William and George, and other close family members.

In bright sunshine, The Mall below was awash with red, white and blue union flags, with some die-hard royal fans camping for days to be in prime position for the display of pomp and pageantry.

The queen's grandson Prince Harry and his American wife Meghan will watch the Trooping the Colour after jetting in from California
The queen's grandson Prince Harry and his American wife Meghan will watch the Trooping the Colour after jetting in from California POOL via AFP / John Stillwell

But many acknowledged the coming end of an era in what some feel could be the last major public event of her long reign.

"We know it's a special occasion because it might be the last day we'll see Her Majesty in a public event," ambulance service worker Gilbert Falconer, 65, told AFP after travelling from Scotland.

Buckingham Palace released a new photograph of the queen to mark her Platinum Jubilee
Buckingham Palace released a new photograph of the queen to mark her Platinum Jubilee BUCKINGHAM PALACE via AFP / Ranald MACKECHNIE
Crowds have been gathering for days on The Mall outside Buckingham Palace
Crowds have been gathering for days on The Mall outside Buckingham Palace AFP / Ben Stansall

Recognition of the queen's unprecedented reign saw tributes from political and religious leaders from across the world, including Pope Francis and US President Joe Biden.

Queen Elizabeth II will make two appearances on the balcony of Buckingham Palace on Thursday
Queen Elizabeth II will make two appearances on the balcony of Buckingham Palace on Thursday POOL via AFP / Andrew Matthews

Even the leader of pro-Irish nationalist party Sinn Fein in Northern Ireland Michelle O'Neill praised the queen's "significant" contribution to the peace process.

O'Neill's letter would have been unthinkable before the 1998 peace deal that ended years of violence over British rule.

Huge crowds converged on central London for the festivities
Huge crowds converged on central London for the festivities POOL via AFP / Daniel LEAL

Sinn Fein was the political wing of the Irish Republican Army paramilitary group, which blew up the queen's cousin, Lord Louis Mountbatten, in 1979.

Elizabeth's heir Prince Charles stood beside his mother on the Buckingham Palace balcony
Elizabeth's heir Prince Charles stood beside his mother on the Buckingham Palace balcony AFP / Daniel LEAL

The queen, a keen horsewoman, has previously taken the salute in person at Horse Guards Parade, the site a short ride down The Mall where king Henry VIII once jousted.

But her 73-year-old son and heir Prince Charles stepped in this year, in the latest public sign of his future role as king.

The queen later helped light th principal beacon of more than 3,000 across Britain and the Commonwealth
The queen later helped light th principal beacon of more than 3,000 across Britain and the Commonwealth POOL via AFP / Adrian DENNIS

Joining him at the parade of the colour or regimental flag of the 1st Battalion Irish Guards was his sister Princess Anne, 71, and his eldest son, Prince William, 39, both in ceremonial uniform.

Senior royals watching on included Charles' younger son, Prince Harry, and his wife Meghan, on a rare visit from California.

They kept a low profile while the queen's disgraced second son, Prince Andrew, 62, was not seen and will also miss Friday's service after contracting Covid.

The queen was at Windsor Castle, west of London, on Thursday night for a ceremony to light the principal beacon of more than 3,000 across Britain and the Commonwealth of 54 nations that she heads.

Elizabeth was a 25-year-old princess when she succeeded her father king George VI in 1952, bringing a rare touch of glamour to a battered nation recovering from World War II.

Seventy years on, she is now the only monarch most Britons have ever known, and an enduring figurehead through often troubled times.

Britain's first and very likely only Platinum Jubilee will see street parties, pop concerts and parades until Sunday and a huge public pageant involving 6,000 performers.

On Wednesday the Queen thanked everyone involved in organising community events in Britain and around the world, and said she was "inspired by the goodwill" shown to her.

"I... hope that the coming days will provide an opportunity to reflect on all that has been achieved during the last 70 years, as we look to the future with confidence and enthusiasm," she added.

The jubilee, held against a backdrop of rising inflation that has left many Britons struggling, is being seen not just as respite for the public after two years hit by Covid, as well as world events.

But it was also a welcome return to normality for the royals.

Harry, 37, and Meghan, 40, caused shockwaves in early 2020 by moving to North America, from where they have publicly criticised royal life.

In April last year, the queen lost her husband of 73 years, Prince Philip, and was forced to sit alone at his funeral due to coronavirus restrictions.

Andrew, who in February settled a US civil claim for sexual assault, has effectively been fired from his royal duties.