Lanterns have been lit at the field in Shanksville, Pennsylvania, where hijacked United Airlines flight 93 crashed on September 11, 2001.
Former Presidents George W Bush and Bill Clinton paid tribute to the 40 passengers and crew who died on the doomed flight – ahead of the 10th anniversary of the Al Qaeda terrorist attacks on the United States.
The jet crashed as air travellers tackled the hijackers after hearing about the dramatic events in New York and Washington.
“What happened above this Pennsylvania field ranks among the most courageous acts in American history,” said former President George W Bush. “The memorial we dedicate today will ensure that our nation always remembers those lost here on 9/11.”
The ceremony drew more than 4000 people including hundreds of victims’ relatives.
Former President Bill Clinton told the audience: “They (the victims) gave the entire country an incalculable gift. They saved the capital from attack. They saved God knows how many lives.”
Later, current President Barack Obama paid tribute to American soldiers killed in Iraq and Afghanistan as part of the war on terror.
He was joined by First Lady Michelle Obama at Arlington National Cemetery.
Later today, Obama will lead America in further commemorations to mark the official 10th anniversary of the terrorist attacks.