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King Charles III will take part in historic Accession Council ceremony, which will be followed by the first public proclamation of the new sovereign.
Thai King Maha Vajiralongkorn has sent a message of condolence to Charles, writing that the Queen had "truly dedicated her life to serving the people".
In a message, he wrote:
Queen Suthida, members of the Thai royal family, and I are deeply saddened by the passing of Her Majesty Queen Elizabeth II. The people of Thailand join with the people of Great Britain in mourning this great loss of the head of state, and together remember Her Majesty's royal grace and royal duties which Her Majesty had carried out as Queen. The people of Thailand and I still remember with much pride and warmth the two historic state visits made by Your Majesty in 1972 and 1996, as guests of my beloved father, His late Majesty King Bhumibol Adulyadej. May I, on behalf of the people of Thailand, express to Your Majesty, the Royal Family and the British people our heartfelt sympathy and condolences.
Queen Suthida, members of the Thai royal family, and I are deeply saddened by the passing of Her Majesty Queen Elizabeth II. The people of Thailand join with the people of Great Britain in mourning this great loss of the head of state, and together remember Her Majesty's royal grace and royal duties which Her Majesty had carried out as Queen.
The people of Thailand and I still remember with much pride and warmth the two historic state visits made by Your Majesty in 1972 and 1996, as guests of my beloved father, His late Majesty King Bhumibol Adulyadej.
May I, on behalf of the people of Thailand, express to Your Majesty, the Royal Family and the British people our heartfelt sympathy and condolences.
Here is the front page of the Guardian's Saturday paper:
Guardian front page, Saturday 10 September 2022: 'I shall endeavour to serve you with loyalty and love' pic.twitter.com/vkbu34xM5T
Today's papers are dominated by King Charles III's emotional address to the nation, which was delivered on Friday evening. Read the Guardian's round up of the front pages here:
Related: 'I pledge myself to you': what the papers said about King Charles's speech
Members of the cabinet will have their first meeting with the King just days after Liz Truss formed her new government, according to Press Association.
It reports:
Senior ministers will attend the Accession Council at St James's Palace where Charles will be formally proclaimed sovereign. Later they will travel to Buckingham Palace for an audience with the monarch. Truss, who was invited to form a Government by the late Queen on Tuesday, had her initial audience with the King on Friday evening. Charles told her: "It was the moment I have been dreading, as I know a lot of people have. We'll try to keep everything going."
Senior ministers will attend the Accession Council at St James's Palace where Charles will be formally proclaimed sovereign.
Later they will travel to Buckingham Palace for an audience with the monarch.
Truss, who was invited to form a Government by the late Queen on Tuesday, had her initial audience with the King on Friday evening.
Charles told her: "It was the moment I have been dreading, as I know a lot of people have. We'll try to keep everything going."
Parliament will also convene today, a rare Saturday sitting, for MPs to share memories of the Queen and approve a message of condolence to the King.
A ceremony to proclaim the accession of King Charles III will also take place on Saturday in Ottawa.
The ceremony will be held at 10 am EDT (1400 GMT) at Rideau Hall, the official residence of the governor-general, the monarch's representative in Canada, Reuters reports.
Canada ceased being a colony of Britain in 1867, but remained in the British Empire until 1982, and is still a member of the Commonwealth.
Charles, 73, automatically became monarch upon the Queen's death on Thursday, but today's proceedings are a constitutional formality to recognise his sovereignty.
Normally such ceremonies would happen within 24 hours of a sovereign's death, but the late announcement of the Queen's death meant it wasn't possible to organise in time for Friday.
Here's some further detail on today's events:
The King will be proclaimed at the Accession Council, which is made up of members of the Privy Council, a group of senior politicians, as well as high-ranking civil servants, Commonwealth high commissioners, and the Lord Mayor of London. (You can find the full details here)
The Accession Council, attended by Privy Councillors, is divided into two parts.
First, the Privy Council, without the King present, will proclaim the sovereign and approve arrangements for his upcoming proclamation. Then, the King will meet with the Privy Council, make his declaration and read and sign an oath.
Afterwards, the Principal Proclamation, declaring Charles as the new king, will be read at 11.00hrs from the balcony overlooking Friary Court at St James's Palace.
In recognition of the new sovereign, flags will be flown at full-mast from the time of the Principal Proclamation at St James's Palace until one hour after the Proclamations in Scotland, Northern Ireland and Wales. Afterwards, flags will return to half-mast in mourning of the Queen.
Related: Charles to be proclaimed King at St James's Palace on Saturday
Charles will be formally proclaimed King at a ceremony at St James's Palace on Saturday morning, with proceedings televised for the first time.
Charles will take part in a historic Accession Council ceremony that will begin at 10am.
Afterwards, the first public proclamation of the new sovereign will be held at 11am. It is traditionally read by the Garter King of Arms in the open air to a trumpet fanfare from the Friary Court balcony at St James's, with gun salutes fired in Hyde Park and at the Tower of London.
A second proclamation at the Royal Exchange in the City of London will be read one hour later, at noon. Separate proclamations will be read in Scotland, Wales and Northern Ireland at noon on Sunday.
During an address to the nation on Friday evening, King Charles III pledged to serve the country "with loyalty, respect and love".
He paid tribute to his mother, the Queen, saying: "May 'flights of angels sing thee to thy rest.'"
Speaking with "feelings of profound sorrow", he said: "Queen Elizabeth's was a life well-lived, a promise with destiny kept, and she is mourned most deeply in her passing. That promise of lifelong service I renew to you all today."
William and Catherine were named Prince and Princess of Wales, and Charles spoke of his love for Harry and Meghan.
The King also met with Prime Minister Liz Truss on Friday evening at Buckingham Palace. When he arrived at the palace, he was met with cheers and kisses from thousands of well-wishers.
Tributes to Queen Elizabeth continued overnight. In Australia, Prime Minister Anthony Albanese praised "her decency, her commitment to service" during a wreath-laying ceremony in Canberra.
Across the world, from New York to Sydney, Berlin to Tel Aviv, cities have projected Elizabeth II's likeness or the colours of the Union Jack in memory of the late Queen.