Queen Elizabeth II, who ruled the UK for 70 years, passed away at Balmoral at the age of 96.
Rest in peace QUEEN Elizabeth
"We deeply mourn the passing of a cherished sovereign and a much-loved mother," he said.
"I know her loss will be felt deeply throughout the country, the realms, and the Commonwealth, as well as by countless people worldwide."
He said he and his family would, near Aberdeen, during the coming period of mourning, after doctors placed the Queen under medical supervision.
Prince William, her grandson and now heir to the throne, and his brother, Prince Harry, were also present.
Obituary: A long life marked by a sense of duty The moment Queen Elizabeth II's death was announced on the BBC, politicians paid tribute to King Charles III, the new monarch.
Prime Minister Liz Truss, who was appointed by Queen Elizabeth II on Tuesday, said the monarch was the rock on which modern Britain was built, who had "provided us with the stability and strength that we needed".
"We offer him our loyalty and devotion, just as his mother devoted so much, to so many, for so long," she said of the new King.
"And, with the passing of the second Elizabethan age, we usher in a new era in the magnificent history of our great country, exactly as Her Majesty would have wished, by saying 'God save the King.'"
The Archbishop of Canterbury, Justin Welby, expressed "profound sadness" for the Church of England, of which the monarch is supreme governor.
"My prayers are with the King and the Royal Family," Media caption,
Queen's death a huge shock to the nation and the world - Prime Minister Truss
Queen Elizabeth II's tenure as head of state spanned post-war austerity, the transition from empire to Commonwealth, the end of the Cold War and the UK's entry into - and withdrawal from - the European Union.
Her reign spanned 15 prime ministers starting with Winston Churchill, born in 1874, and including Ms Truss, born 101 years later in 1975.
She held weekly audiences with her prime minister throughout her reign.