Political leaders, members of the public and well-wishers around the world have paid tribute to Prince Philip, Queen Elizabeth II's husband, after his death at the age of 99.
A statement issued by Buckingham Palace just after midday spoke of the Queen's "deep sorrow" following his death at Windsor Castle on Friday morning.
He was the longest-serving royal consort in British history.
Prince Charles said his father's life was an "astonishing achievement".
Reflecting on Prince Philip's life for a BBC programme, Prince Charles said: "His energy was astonishing in supporting my mama [the Queen] - and doing it for such a long time, and, in some extraordinary way, being able to go on doing it for so long.
"What he has done has amounted to an astonishing achievement, I think."
Princess Anne said the duke "treated everyone as an individual, and gave them the respect he felt they were due as individuals".
Announcing the duke's death, the Palace said: "It is with deep sorrow that Her Majesty The Queen announces the death of her beloved husband.
"The Royal Family join with people around the world in mourning his loss."
It is understood that the Prince of Wales travelled to Windsor Castle to visit his mother on Friday afternoon.
Speaking at Downing Street, prime minister Boris Johnson added that the duke had "earned the affection of generations here in the United Kingdom, across the Commonwealth, and around the world".
Meanwhile, Justin Welby, the Archbishop of Canterbury, said he "consistently put the interests of others ahead of his own and, in so doing, provided an outstanding example of Christian service".
In tribute to the duke, Westminster Abbey tolled its tenor bell once every 60 seconds for 99 times from 18:00 BST - to honour each year of his life.
Earlier, the flag at Buckingham Palace was lowered to half-mast and a notice was posted on the gates to mark the duke's death.
People placed floral tributes outside the central London landmark, while hundreds visited Windsor Castle to pay their respects.
However, the government urged the public not to gather or leave tributes at royal residences amid the coronavirus pandemic.
The Royal Family has asked people to consider making a donation to a charity instead of leaving flowers in memory of the duke, and an online book of condolence has been launched on the official royal website for those who wish to send messages.
A message on the website of the Duke and Duchess of Sussex's non-profit organisation Archewell paid tribute to the "loving memory" of the Duke of Edinburgh, saying: "Thank you for your service... you will be greatly missed."
From midday on Saturday, a 41-gun salute will take place for Prince Philip in cities including London, Edinburgh, Cardiff and Belfast, as well as in Gibraltar and at sea from Royal Navy warships, the Ministry of Defence said. They will be broadcast online and on television for the public to watch from home.
All UK government buildings have been told to fly official flags at half-mast in tribute to the duke until 08:00 BST on the day after the duke's funeral.
Reaction to death of Prince Philip
Prince Philip's funeral will take place at St George's Chapel, Windsor - but the arrangements have been amended in light of the coronavirus pandemic, the College of Arms said in a statement.
The duke will not have a state funeral and there will be no lying-in-state, in line with his wishes, it added.
The college said the duke will, however, lie at rest in Windsor Castle before a royal ceremonial funeral, which means his death will be marked in the same way as the Queen Mother.
The public are "regretfully" requested not to attend due to the pandemic, and it is understood the Queen is considering modified funeral and ceremonial arrangements.
Politicians across the UK were united in mourning following the announcement of the duke's death.
Labour leader Sir Keir Starmer said the UK had "lost an extraordinary public servant", while Scotland's First Minister Nicola Sturgeon said his "long contribution to public life in Scotland will leave a profound mark on its people".
Wales' First Minister Mark Drakeford said the duke "served the crown with selfless devotion and generosity of spirit", and Northern Ireland's First Minister Arlene Foster said he was "widely respected for his active and dedicated service to the country".
Parliament will honour the duke on Monday, with the House of Commons sitting at 14:30 BST for tributes following his death.
Parties have also suspended their campaigning for elections on 6 May, which will see voters head to the polls for council and mayoral positions in England, the Scottish Parliament and Welsh Parliament.
Meanwhile, Commonwealth leaders led international reaction to the duke's death.
Australian Prime Minister Scott Morrison tweeted that the duke "embodied a generation that we will never see again", while Canada's Prime Minister Justin Trudeau described Philip as a "man of great purpose and conviction".
And Indian Prime Minister Narendra Modi recalled the duke for his "distinguished career in the military" and work "at the forefront of many community service initiatives".
US President Joe Biden and First Lady Jill Biden joined other world leaders in sending their "deepest condolences" to the Queen, the Royal Family and the people of the UK.
In a statement, they said the duke "gladly dedicated himself to the people of the UK, the Commonwealth, and to his family".
"His legacy will live on, not only through his family, but in all the charitable endeavours he shaped," it said.
Prince Philip and the Queen had four children, eight grandchildren and 10 great-grandchildren.
Their first son, the Prince of Wales, Prince Charles, was born in 1948, followed by his sister, the Princess Royal, Princess Anne, in 1950, the Duke of York, Prince Andrew, in 1960 and the Earl of Wessex, Prince Edward, in 1964.
Prince Philip was born on the Greek island of Corfu on 10 June 1921.
His father was Prince Andrew of Greece and Denmark, a younger son of King George I of the Hellenes.
His mother, Princess Alice, was a daughter of Prince Louis of Battenberg and a great-granddaughter of Queen Victoria.