King Charles and the Queen Consort shook hands and talked with members of the public as crowds gathered for their historic visit to Northern Ireland.
The monarch went on a brief walkabout in Hillsborough shortly after landing in Belfast for what is the first visit to the country by a British king in nearly 80 years.
King Charles has now visited 40 times but this is his first time as monarch.
His Majesty and his wife Camilla are now on their way to Belfast City Airport to fly to London where the monarch will receive his mother's coffin at Buckingham Palace.
They ended their visit to Northern Ireland with a service of reflection for the Queen's life at St Anne's Cathedral in Belfast.
British Prime Minister Liz Truss, Irish President Michael D Higgins, Taoiseach Micheal Martin and Foreign Affairs Minister Simon Coveney were also at the memorial service.
King Charles and the Queen Consort met leaders from all the major faiths in Northern Ireland at the cathedral.
His Majesty and Camilla earlier met Northern Ireland's political leaders, including the vice president of Sinn Fein, at Hillsborough Castle.
Hundreds of people gathered in Royal Hillsborough ahead of the King's arrival, with many lining the village's Main Street near the castle.
Spectators were driven into the village on shuttle buses amid a massive security operation.
The King and Queen Consort held a private audience with the new Northern Ireland Secretary Chris Heaton-Harris shortly after arriving at the castle.
They then received a message of condolence on behalf of the country's people from the speaker of Stormont Assembly Alex Maskey at a reception at the castle.
King Charles pledged to "seek the welfare" of all Northern Ireland's people and spoke of his mother and the peace process in his reply.
He said: "In the years since she began her long life of public service, my mother saw Northern Ireland pass through momentous and historic changes.
"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.
"My mother felt deeply, I know, the significance of the role she herself played in bringing together those whom history had separated, and in extending a hand to make possible the healing of long-held hurts."
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The royal couple later met with political leaders including Sinn Fein's vice president Michelle O'Neill.
The meeting highlights the rapid evolution of Irish republican politics over the last decade.
Back in 2011, Sinn Fein was widely criticised for boycotting the Queen's state visit to Ireland. It was the first state visit by a British monarch since Ireland won independence from Britain a century ago.
Members of the public were invited to line the streets between the castle and the cathedral before King Charles and Camilla travelled along Main Street and Lisburn Street in Hillsborough, before heading to Belfast along Wellington Place, Donegall Square North, Chichester Street and Victoria Street.
The King and Queen Consort will visit Writers' Square in Belfast after the memorial service.
The Northern Ireland visit began when the royal couple were greeted by a group of dignitaries at Belfast City Airport earlier this afternoon.
Lord Lieutenant of Belfast Fionnuala Jay-O'Boyle and Belfast City Airport chief executive Matthew Hall were present as the King and Queen Consort left their plane just after noon.
Ella Smith and Lucas Watt, both 10, of Forge Integrated Primary School in Belfast, presented the King with a gift of truffles with an image of the Giant's Causeway on the tin, while Camilla was given a small posy of flowers taken from the gardens of Hillsborough Castle.
King Charles asked the two children: "Are you enjoying your day off school?"
The Queen's coffin will make a poignant journey to Buckingham palace from St Giles' Cathedral in Edinburgh.
Members of the public are already queueing for the Queen's lying in state at Westminster Hall in London, which opens on Wednesday, and thousands are still placing flowers and tributes in Green Park.