With flags fluttering at half-mast, the United States has paused to mourn President John F Kennedy and a generation’s shattered dreams, cut down 50 years ago by an assassin’s bullet.

The young leader’s brutal death shocked the world.

Five decades on the wound is still raw, with many still obsessed by the conspiracy theories surrounding his death, and others gripped by regret for the America they imagine might have been.

Across the nation, at ceremonies large and small, many took comfort in reflecting upon the words of a charismatic man whose soaring rhetoric and call to service continues to inspire.

“Today, we honour his memory and celebrate his enduring imprint on American history,” President Barack Obama declared.

The ringing of church bells filled Dealey Plaza at 12:30 pm as a moment of silence marked the time that the deadly shots burst forth from the sixth floor of a book depository in Dallas, Texas.

“You, Lord, have lifted us up from the horrible tragedy enacted in this place … the gun shot by one man that killed a president in whom many of us had set our hopes and dreams for a better America,” intoned Bishop Kevin Farrell of the Catholic Diocese of Dallas.

A steady stream of mourners visited Kennedy’s tomb in Arlington National Cemetery outside Washington, where two kilted pipers from the Black Watch of the British army repeated a tribute their regiment performed at his funeral 50 years ago.

Across the Atlantic too, Kennedy was remembered.

A wreath-laying ceremony was held in the Berlin neighbourhood where Kennedy gave his famed Cold War-era “Ich bin ein Berliner” speech to a rapturous crowd.

In a proclamation ordering flags be lowered at government buildings and even private homes, Obama recalled Kennedy’s leadership in the Cuban missile crisis, his speech in Berlin and his drive to advance the rights of African Americans and women.

“Today and in the decades to come, let us carry his legacy forward,” Obama wrote Thursday.

“Let us face today’s tests by beckoning the spirit he embodied – that fearless, resilient, uniquely American character that has always driven our nation to defy the odds, write our own destiny, and make the world anew.”

Cut down in his first term at the age of 46 as he was driven through Dallas in an open-top limousine on November 22, 1963, Kennedy’s unfulfilled promise has become a symbol of the lost nobility of politics.

Despite the many scandals that have since become attached to his name, he is seen as a president who enlisted his fellow countrymen in lofty goals – like putting a man on the Moon – “not because they are easy, but because they are hard.”

He was the fourth US president to be killed in office, but the first whose death was caught on film. News of his death flashed around the world by television.

Many refuse to believe the assassination could be the act of a single man: troubled Marine Corps veteran turned Soviet defector Lee Harvey Oswald, 26, who was shot to death on live television two days later as he was being transferred to the county jail.

Conspiracy theories continue to captivate doubters and fuel an industry of books, films and television specials.