David Cameron has said the thoughts of Britain are with the American people as they prepare to mark the 50th anniversary of the assassination of President John F Kennedy - who, he said, represented "the very best of politics".
It is exactly half a century since the shooting of JFK as he rode in a motorcade with wife Jacqueline through Dallas, Texas - an event which resonates to this day and has spawned numerous conspiracy theories.
Last night, the Prime Minister said: " Fifty years ago John F Kennedy lost his life - and the world lost an inspiration.
"Wherever you go in the world today, the three letters 'JFK' are instantly recognisable. They summon up the very best of politics: energy, optimism, hope - the belief that a nation united can achieve almost anything.
"It was these ideals which came to define the Kennedy presidency. He demanded that his country rise to the challenges of its time - and the people responded in kind.
"Although his presidency was tragically cut short, its legacy was felt long after. Civil rights, a man on the Moon - both achieved after his death, but only possible because of the leadership shown during his life.
"Tomorrow will be a day to remember with gratitude what President Kennedy's life still teaches us - and our thoughts are with the American people."
Earlier, Barack Obama paid his respects and laid a wreath at his Democratic predecessor's grave in Arlington National Cemetery in Washington.
Mr Obama awarded the Presidential Medal of Freedom - an award inaugurated by Mr Kennedy shortly before he lost his life - to a number of significant figures including ex-president Bill Clinton.
Today, he is due to meet at the White House with leaders and volunteers from Peace Corps programme, which was established by President Kennedy.
Commemorations are taking place in cities across the US, including Dallas, where church bells will toll.