King Charles III on Friday readied to address his mourning subjects on the first full day of his new reign, as Britain and the world commemorated the extraordinary life of his late mother, Queen Elizabeth II.

At 73, Charles is the oldest monarch yet to ascend the throne, following the death of his "cherished" mother at her remote Scottish estate on Thursday.

He was due to return to London from Balmoral, where the 96-year-old queen died "peacefully" after a year-long period of ill-health and decline, at the culmination of a record-breaking reign of 70 years.

"During this period of mourning and change, my family and I will be comforted and sustained by our knowledge of the respect and deep affection in which the queen was so widely held," Charles said in a statement.

Buckingham Palace said the king and other members of the royal family would observe an extended mourning period from now until seven days after her funeral.

The date of the funeral, which will be attended by heads of state and government, has yet to be announced but is expected to be on Monday, September 19.

- King's speech -

One of the planet's most recognisable people, the queen was the only British monarch most people alive today had ever known.

The tributes were universal, including from Russia and China.

New York's Empire State Building was illuminated after sunset in silver and royal purple, while the Eiffel Tower in Paris dimmed its lights in tribute.

US President Joe Biden described the queen, whom he met for tea at Windsor Castle last year, as "a stateswoman of unmatched dignity".

He relayed the comforting words she gave when the United States was plunged into mourning after the September 11, 2001 attacks.

The Daily Telegraph newspaper chose the same words for its sombre front-page headline: "Grief is the price we pay for love," it read.

Other British newspapers also printed special editions to mark the occasion. "Our hearts are broken," headlined popular tabloid the Daily Mail.

The Mirror wrote simply: "Thank you."

Charles's inaugural address, set to be pre-recorded, was expected to be broadcast on Friday evening, part of 10 days of plans honed over decades by Buckingham Palace and the UK government.

The new king was also expected to hold his first audience with Prime Minister Liz Truss, who was only appointed on Tuesday in one of the queen's last official acts before her death.

Truss acclaimed the "second Elizabethan age", five centuries after the celebrated first.

"We offer him (Charles) our loyalty and devotion just as his mother devoted so much to so many for so long," she said in a televised address Thursday. "God save the king."

Charles was also due to meet officials in charge of the arrangements for his mother's elaborate state funeral, which will take place before she is laid to rest in the King George VI memorial chapel at Windsor Castle.

Gun salutes -- one round for every year of the queen's life -- will be fired Friday across Hyde Park in central London and from the Tower of London on the River Thames.

Muffled church bells will toll at Westminster Abbey, St Paul's Cathedral and at Windsor, among other places, and Union flags were flying at half-mast across the UK.

Truss and other senior ministers were set to attend a public remembrance service at St Paul's, while the UK parliament will start two days of special tributes.

The queen's death and its ceremonial aftermath comes as the government strives to rush through emergency legislation to tackle the kind of war-fuelled economic privation that marked the start of Elizabeth's reign in 1952.

- Tributes -

Elizabeth's public appearances had become rarer in the months since she spent an unscheduled night in hospital in October 2021 for undisclosed health tests.

She was seen smiling in her last official photographs from Tuesday when she appointed Truss as the 15th prime minister of her reign, which started with Winston Churchill in Downing Street.

But the queen, visibly thinner and stooped, leant on a walking stick. Her hand was also bruised dark blue-purple, sparking concern.

Jane Barlow, the photographer who took the last public pictures of the queen on Tuesday, said she was "frail" but in "good spirits".

"I got a lot of smiles from her," said Barlow, who works for the UK's domestic Press Association news agency.

The queen's closest family members had rushed to be at her bedside at Balmoral, a private residence set among thousands of acres (hectares) of rolling grouse moors and forests in the Scottish Highlands.

Her body is expected to remain there initially before being taken Sunday to the Palace of Holyroodhouse in Edinburgh.

From the Scottish capital, her coffin is due to be flown to London on Tuesday for a lying in state accessible to the public.

Officials expect more than one million people to file past the catafalque in Westminster Hall, the oldest part of the parliamentary complex, before the televised funeral service at Westminster Abbey opposite.

Braving steady rain, crowds gathered late into Thursday night outside Buckingham Palace in London, and Windsor Castle west of the capital, placing flowers and reflecting on her long reign.

Londoner Joshua Ellis, 24, choked back tears as he mourned the "nation's grandmother" at the palace.

"I know she is 96 but there is still a sense of shock. She is in all our minds and hearts," he said.

"You could always look to the queen, to a sense of stability. Every time people needed support, she was there."

As day broke on Friday, Joan Russell, a 55-year-old project manager from Hackney, northeast London, had tears running down her cheeks as she looked at the flowers outside the palace.

"I think I came to say a prayer. She has been our monarch all my life and she has led by example, she has learnt, she has listened, wherever you go, she is our stamp," she told AFP.

"Charles has had such a great example to follow. I believe he will do his very utmost to continue the legacy of his parents -- his mum and dad -- have set before him."

- Consistently popular -

Queen Elizabeth II came to the throne aged just 25 in the exhausted aftermath of World War II, joining a world stage dominated by political figures from Churchill to Mao Zedong and Joseph Stalin.

In the ensuing decades, the last vestiges of Britain's vast empire crumbled.

At home, Brexit shook the foundations of her kingdom, and her family endured a series of scandals.

But throughout, she remained consistently popular and was head of state not just of the United Kingdom but 14 former British colonies, including Australia and Canada.

New Zealand proclaimed Charles its new king. But Australia's new government looks set to revive a push to ditch the monarchy, casting doubt on his inheritance even as it mourns the queen.

The final public farewell at Westminster Abbey in London will be a public holiday in the form of a Day of National Mourning.

Charles's coronation, an elaborate ritual steeped in tradition and history, will take place in the same historic surroundings, as it has for centuries, on a date to be fixed.

On Saturday, his reign will be formally proclaimed by the Accession Council, which comprises senior politicians, bishops, City of London dignitaries and Commonwealth ambassadors.