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.
His Majesty later said the Queen “never ceased to pray for the best of times” for Northern Ireland in a short speech at Hillsborough Castle.
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.
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His Majesty and his wife Camilla are now at St Anne’s Cathedral for a service of reflection for the Queen’s life.
British Prime Minister Liz Truss, Irish President Michael D Higgins, Taoiseach Micheal Martin and Foreign Affairs Minister Simon Coveney are at the memorial service.
Meanwhile, His Majesty and the Queen Consort are due to meet leaders from all the major faiths in Northern Ireland at the cathedral.
King Charles and the Queen earlier met Northern Ireland’s political leaders, including the vice president of Sinn Fein, at Hillsborough Castle.
King Charles and Queen Camilla arrive at Hillsborough Castle
Hundreds of people have gathered outside 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 speaks after the message of condolence
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 to the message.
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.
Flowers and tributes have been laid outside Hillsborough Castle
Members of the public were invited to line the streets between the castle and the cathedral as King Charles and Camilla will travel 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 also visit Writers’ Square in Belfast, which will be closed to the public.
The royal couple were greeted by a group of dignitaries including the new Northern Ireland Secretary Chris Heaton-Harris at Belfast City Airport after their flight from Edinburgh.
King Charles III waves to onlookers after arriving at Belfast City Airport
Lord Lieutenant of Belfast Fionnuala Jay-O’Boyle and Belfast City Airport chief executive Matthew Hall were also 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 monarch and the Queen Consort, who are on their Operation Spring Tide tour around the UK, will travel back to London later where the monarch will receive his mother’s coffin at Buckingham Palace.
Meanwhile, the Queen’s coffin will make a poignant journey to the 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.