DALLAS (AFP) - With flags fluttering at half-staff, the United States paused on Friday to mourn President John F. Kennedy and a generation's broken dreams, cut down 50 years ago by an assassin's bullet.

The young leader's brutal televised death, a dark turning point even in an era gripped by the Cold War nuclear stand-off and bloodshed in the jungles of Vietnam, shocked a global audience of millions.

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.

Across the Atlantic too, Kennedy was remembered.

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

At Kennedy's tomb in Arlington Cemetery outside Washington, two kilted pipers from the Black Watch of the British army repeated a tribute their regiment had performed at his funeral 50 years ago.

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 on 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."