Britain's longest reigning monarch Queen Elizabeth II, died on September 8 at her Scottish estate after a long-drawn battle with health problems since last year
"The death of my beloved Mother, Her Majesty The Queen, is a moment of the greatest sadness for me and all members of my family," the new king, her eldest son King Charles III, said.
"I know her loss will be deeply felt throughout the country, the Realms and the Commonwealth, and by countless people around the world," the 73-year-old said in a statement.
Thousands gathered outside Buckingham Palace, in central London, and there was a stunned silence when the flag was lowered to half-mast.
There were similar scenes outside the queen's Windsor Castle home. Black cabs lined up outside the palace to pay homage to the Queen.
On Elizabeth's, Charles automatically became monarch of the United Kingdom and the head of state of 14 other realms including Australia, Canada and New Zealand.