A pair of Democratic presidents, Barack Obama and Bill Clinton silently honored John F. Kennedy at the former US leader's grave Wednesday, marking the 50th anniversary of his assassination.

Obama and Clinton, along with First Lady Michelle Obama and former secretary of state Hillary Clinton, laid a large blue and white wreath on the sun-dappled grave of JFK, who was gunned down in Dallas on November 22, 1963.

They then stood together, with hands on hearts, as a bugler played the US military lament "Taps" before observing a moment of silence.

Extended members of the Kennedy clan looked on at the tableau of presidential power -- past, present, and possibly future.

The observance came after Obama awarded Bill Clinton and 15 other luminaries of the arts, sport, science and innovation the highest honor for US civilians, the Presidential Medal of Freedom, which was first minted by Kennedy.

Later Wednesday, Obama will pay tribute to Kennedy's legacy at a speech at the Smithsonian Institution in Washington honoring medal awardees.

The Kennedy grave and eternal flame is in Arlington National Cemetery outside Washington on a hillside with a paved area fashioned from Cape Cod granite quarried from near the Kennedy clan's home base in Massachusetts.

The eternal flame was lit by Kennedy's wife Jacqueline Kennedy during his funeral in 1963 and she was buried beside her husband after her own death in 1994.

The poignant moment of remembrance came two days before the official half-century anniversary of the death of Kennedy, who was gunned down in an open-top limousine in Dallas, Texas, in a crime which traumatized the world.

The ceremonies have sparked a prolonged period of national and media reflection on the unfinished legacy of Kennedy, his tragedy-crossed family and of the evocative period in the early 1960s when his political star illuminated the world.

Kennedy's closest living relative, his daughter Caroline, however, was not at Wednesday's ceremony. An early supporter of Obama's presidential ambitions, she has just set off on a new chapter of her life as the US ambassador to Tokyo.

The joint Obama-Clinton appearance at the grave site represented the latest show of unity between two political power families who waged a bitter 2008 Democratic presidential nominating duel.

Hillary Clinton is now the red-hot favorite to land the Democratic nomination for the 2016 election -- but has not said whether she will make another run for the White House.

Presidents Clinton and Obama, two-term leaders both, laid claim to the legacy of John F. Kennedy in their own White House runs.

Clinton was famously pictured meeting Kennedy at an event in the White House Rose Garden in July 1963, and has reminisced about how he set eyes on the presidency himself after shaking JFK's hand.

Obama, who was two years old when the 35th US president was killed, accepted Kennedy's torch of Democratic Party idealism in a key moment in the 2008 campaign -- which irked the Clintons -- when president Kennedy's late brother, senator Edward Kennedy endorsed Obama at American University in Washington.

The two presidents stood together at a painful political moment for Obama, when he may be looking for political inspiration, after being brought low by the botched implementation of his signature health care law.

A CBS News poll published Wednesday found that the president's approval rating was down to 37 percent -- his lowest ever.

In the Medal of Freedom ceremony, Obama said that Clinton's presidency proved that it was possible to grow the economy, cut the deficit and invest in science, technology and education -- remarks which mirrored his own core political argument to American voters.

Kennedy's killing was blamed on a gunman, Lee Harvey Oswald, who was said to be acting alone.

But the 50 years since have been replete with conspiracy theories centering on whether Oswald was the true culprit and if he was acting on his own initiative or was part of a wider plot.