Charles, 73, became monarch immediately after the death of his mother at her Scottish Highland retreat on Thursday, sparking tributes at home and abroad.
Details of his inaugural address are not immediately released by Buckingham Palace. He is due to return to London from Balmoral, where she died “peacefully” after a year of ill health and decline.
Also on Friday, the new king is expected to hold his first audience with Prime Minister Liz Truss. 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.
Gun salutes – one round for every year of the queen’s life – will be fired across Hyde Park in central London and from the Tower of London, the ancient royal fortress on the River Thames.
He will decide on the length of the royal household’s period of mourning, which is expected to last a month, while the UK government will observe 10 days of official remembrance, when limited business is conducted.
British newspapers were inevitably dominated by the queen’s passing, printing special editions to mark the occasion.
“Our hearts are broken,” headlined popular tabloid the Daily Mail. “We loved you Ma’am,” said The Sun. The Mirror wrote simply: “Thank you.”
Elizabeth, whose public appearances had become rarer, died following months of ill-health that first came to light after she spent an unscheduled night in hospital in October 2021 for undisclosed health tests.
In one of her last official acts, on Tuesday she appointed Truss as the 15th prime minister of her reign, which started with Winston Churchill in Downing Street
She was seen smiling in photographs but looked frail and leant on a walking stick. Her hand was also bruised dark blue-purple, sparking concern.
Her coffin will initially remain at Balmoral, a private residence set among thousands of acres (hectares) of rolling grouse moors and forests.
Her closest family members had rushed to be at her bedside at Balmoral, where her body will now lie before being taken to the Scottish capital Edinburgh.
From there, it is expected to travel by train to London for a lying in state and funeral.
Speaking on the steps of Downing Street shortly after her death was announced, 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 the televised address. “God save the king.”
Braving steady rain, crowds gathered late into Thursday night outside Buckingham Palace in London, and Windsor Castle west of the capital, with the number of well-wishers set to swell in the days ahead.
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.”
Almost every billboard in London has Queen Elizabeth on it. Tomorrow the capital will become the focus as King Charles moves to London and addresses the nation.
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 Winston Churchill to Mao Zedong and Joseph Stalin.
Her record-breaking reign straddled two centuries of seismic social, political and technological upheaval.
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.
Britain’s mourning will culminate in a final public farewell at Westminster Abbey in London. The funeral day 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.
In a statement, the new king described his mother as a “cherished sovereign” whose loss would “be deeply felt throughout the country, the Realms and the Commonwealth, and by countless people around the world”.
“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.