The King was joined by his siblings and both his sons to witness the late Queen’s coffin arriving at Buckingham Palace after pledging to follow his mother’s “shining example” in Northern Ireland.
Charles and his Queen Consort were joined by the late monarch’s children and grandchildren and their partners, including the Prince and Princess of Wales and Duke and Duchess of Sussex, at the grand entrance of the royal residence affectionately known as the “office”.
The Queen’s coffin will rest in the bow room before the King leads Wednesday’s procession behind the gun carriage carrying his mother’s coffin to Westminster Hall, where the Queen will lie in state for four days before the state funeral on September 19.
The hearse slowed down as it approached the palace gates before well-wishers cheered “hip hip hooray” after the coffin drove under the arch.
Many people lining the street put down their umbrellas as a sign of respect while some could be seen wiping tears from their eyes.
Phone camera lights lit up the crowds as people documented the historic moment.
The Princess Royal said it had been “an honour and a privilege” to accompany the Queen on her final journeys as she travelled with the monarch’s coffin back to London.
“Witnessing the love and respect shown by so many on these journeys has been both humbling and uplifting,” Anne said.
“We may have been reminded how much of her presence and contribution to our national identity we took for granted.
“To my mother, The Queen, thank you.”
Earlier, the King pledged to “seek the welfare” of all Northern Ireland’s people and described how his family have felt their “sorrows” as he praised his mother’s relationship with Northern Ireland.
Charles, who in 2015 made a pilgrimage to the site of his great-uncle Lord Mountbatten’s murder in an IRA bombing, said the Queen had “never ceased to pray for the best of times for this place and its people”.
Speaking at Hillsborough Castle in Co Down, the royal residence in Northern Ireland, the new monarch said the late Queen was aware of her position in bringing together divided communities “whom history had separated”.
With his Queen Consort watching, the King said: “Through all those years, she never ceased to pray for the best of times for this place and for its people, whose stories she knew, whose sorrows our family had felt, and for whom she had a great affection and regard.”
He added: “Now, with that shining example before me, and with God’s help, I take up my new duties resolved to seek the welfare of all the inhabitants of Northern Ireland.”
After leaving Edinburgh for Belfast on Tuesday morning, Charles and Camilla’s jet touched down at George Best Belfast City Airport, where schoolboy Lucas Watt, 10, from the local cross-community Forge Integrated primary school, presented the King with a tin featuring an image of the famous Giant’s Causeway.
Camilla received a posy of flowers, from schoolgirl Ella Smith, 10, with the blooms taken from Hillsborough Castle.
The King and his Queen Consort shook hands with some of the hundreds of flag-waving people who had waited hours to catch a glimpse of the monarch.
Shouts of “God save the King” were heard and at one moment and a corgi in the crowd snuggled up to Charles when its owner held it up during the walkabout by the royal couple.
A round of applause broke out as they inspected floral tributes to the late Queen before entering the grounds of the castle.
The King also held a private audience with Northern Ireland Secretary Chris Heaton-Harris as well as meeting representatives of political parties in the region.
Invitations to the Queen’s state funeral were also sent out on Monday – but not to Russia, Belarus or Myanmar, while Iran will only be represented at an ambassadorial level, Whitehall sources said.
The event, which follows the lying in state, will see around 500 dignitaries from around the world descend on London to pay their last respects to the long-reigning monarch.
They will join members of the royal family, UK prime ministers past and present, and key figures from public life at Westminster Abbey – the historic church which can hold around 2,000 people – at 11am next Monday.
“This is the biggest international event we have hosted in decades,” a Whitehall source said.
And before dawn, a full rehearsal for the procession of the Queen’s coffin to Westminster Hall took place in central London.
Thousands of soldiers in ceremonial uniform gathered at Buckingham Palace in the early hours of Tuesday for the practice run.
The early-morning rehearsal saw the horse-drawn carriage of the King’s Troop Royal Horse Artillery being led along the route.
Many of central London’s streets were sealed off for the massive operation.
The real procession begins at 2.22pm on Wednesday, and it will last 38 minutes.
King Charles III, members of the royal family and senior staff of the Queen and King’s households are expected to walk behind the coffin.