Britain's longest-reigning monarch died peacefully, surrounded by her family, at Balmoral in Scotland on Thursday.
The King is on his way to London, where he is also due to meet Prime Minister Liz Truss later.
Gun salutes will be fired and church bells will toll on Friday as the UK pays tribute to her reign.
There have been spontaneous gatherings and outpourings of emotion at Balmoral, Buckingham Palace and Windsor.
Members of the public have travelled to leave flowers, messages of thanks and condolence for the Queen and Royal Family, with many visibly tearful or overcome with emotion.
Union jacks are being flown at half mast and Parliament will gather later to pay tribute to her momentous 70-year reign.
The bells of St Paul's Cathedral, Westminster Abbey and Windsor Castle are expected to toll at noon in tribute to her life and service.
A gun salute in London's Hyde Park has been arranged for the following hour, with 96 rounds to mark each year of her life to fire around 13:00 BST.
There will be a remembrance service at St Paul's at 18:00 BST, attended by Prime Minister Liz Truss and other senior ministers.
It will be open to the public, with 2,000 tickets to be released on a first-come-first-served basis.
Those wishing to attend must visit in person to the City of London tourism office on Carter Lane in London to collect a wristband from 11:00
The King and his wife, Camilla, now Queen Consort, are heading to London, where the monarch is expected to address the nation after holding an audience with the new Prime Minister Liz Truss.
All of the Queen's children and grandchildren, the Duke of Cambridge and Duke of Sussex, travelled to Balmoral, near Aberdeen, on Thursday after the Queen's doctors became concerned about her health.
Prince Harry left Balmoral on Friday morning to travel to Aberdeen airport, where he was seen placing an arm around a member of staff.
On Friday, the palace released some details of plans over the coming days, with King Charles declaring a period of Royal mourning be observed for seven days after the funeral of his mother.
There will be no physical book of condolences for members of the public to sign, but the palace has opened an online book of condolences for those who wish to leave messages.
The government has said it expects large crowds to gather in central London and other Royal Residences as a mark of respect, warning there could be some travel disruption, traffic delays and significant crowding,
Members of the public have been asked to leave their flower tributes in designated areas at Royal Residences.
Tributes to the Queen will also be paid by MPs and peers in the Houses of Commons and Lords from midday until Friday evening, with normal politics put on hold.
The Cabinet met on Friday morning, with the only item on the agenda to pay tribute to the Queen.
There will also be a rare Saturday sitting of the House of Commons, where senior MPs will gather to take an oath of allegiance to the new King from 14:00 BST, with condolences continuing again until the evening.