Queen Elizabeth's husband Prince Philip, the man she once called her 'strength and stay', died on Friday (April 9) at Windsor Castle aged 99.
He was known for modernizing the monarchy, and steering the British royal family through repeated crises during his service.
The Duke of Edinburgh, as he was officially known, was by his wife's side throughout her 69-year reign, the longest in British history.
Flags were lowered to half-mast at Buckingham Palace, and at government buildings across London, as the public began to lay flowers outside royal residences.
Prime Minister Boris Johnson also paid his respects to the Queen's consort.
"We mourn today with Her Majesty the Queen, we offer our condolences to her and to all her family, and we give thanks as a nation and a kingdom for the extraordinary life and work of Prince Philip, Duke of Edinburgh."
Many other world leaders have also paid tribute, including U.S. President Joe Biden.
Philip, a former naval officer and Greek prince, had earned a reputation for a tough, no-nonsense attitude, which earned him respect from many Britons.
But he was also known for off-the-cuff remarks that sometimes caused offense.
He was a favorite to newspaper editors, keen to pick up on any stray remarks at official events.
Philip spent four weeks in hospital earlier this year for treatment for an infection, and to have a heart procedure, but he returned to Windsor in early March.
He died just two months before his 100th birthday.