America paid tribute to the "forgotten heroes" of 9/11 on Saturday with the solemn dedication of a national memorial to the 40 passengers and crew of United Airlines Flight 93.

Polished granite slabs bearing the names of those who struggled with Al-Qaeda terrorists on the fourth airliner hijacked on September 11, 2001 were unveiled in the Pennsylvania countryside where the Boeing 757 crashed.

Then-president George W. Bush, his predecessor Bill Clinton and current Vice President Joe Biden joined families of the victims and hundreds of others -- many in patriotic T-shirts or holding US flags -- under a slate gray sky.

"One of the lessons of 9/11 is that evil is real -- and so is courage," Bush said, recalling "the first counter-offensive in the war on terror (and) one of the most courageous acts in American history."

"The temptation of isolation is deadly wrong," added Bush, clearly directing his remarks to those who favor less US engagement in world affairs.

It was up to the United States, he said, to "lead the cause of freedom... a world of dignity, liberty, and hope would be better and safer of all."

Bells struck by National Parks Service rangers in stetson hats tolled as the names of the dead were read aloud, and Canadian songstress Sarah McLachlan, alone at the piano, performed her mournful "I Will Remember You."

Later, many families -- sometimes with young children -- trekked gingerly across the rain-sodden field to lay wreaths and remember loved ones at the very spot -- marked by a 17-ton boulder -- where Flight 93 slammed into the ground.

On Sunday, President Barack Obama is to join a two-hour commemorative service at the spot where Flight 93 went down -- lifting the profile of a sometimes overlooked episode of the catastrophic 9/11 attacks.

The Flight 93 National Memorial currently includes an elongated walkway that sweeps past a circular field -- permanently restricted to family members -- where the jet crashed.

The adjoining granite wall bearing the names of the dead retraces the direction in which Flight 93 came down. Planted by the entry to the walkway are three young elm trees, representing the three 9/11 sites.

Future plans call for a memorial wall by 2014, a grove of 40 trees and, in time, a 93-foot (28-meter) "tower of voices" comprising 40 wind chimes -- although 10 million dollars still needs be raised before the project is done.

"We built the Statue of Liberty and that was important, but there's nothing more important that this memorial," said Ed Rendell, governor of Pennsylvania during the period when the idea of a lasting memorial took hold.

Several days of heavy rain that triggered floods in much of Pennsylvania forced organizers to alter the staging of this weekend's events, until grey skies gave way to sunshine during the ceremony.

Notable upon the stage were the flags of Germany, Japan and New Zealand -- in remembrance of wine merchant Christian Adams, 37, student Toshiya Kuge, 20, and lawyer Alan Anthony Beaven, 48, the non-native-born Americans on the flight.

A US Navy brass quintet in crisp white uniforms played a prelude. FBI agents raised the national flag. Award-winning bagpiper Bruce Liberati played "Amazing Grace" despite a flagging sound system.

Ethel Stevanus, who lives near Shanksville, came Saturday in a T-shirt emblazoned "Let's Roll" -- the battle cry that software executive Todd Beamer reputedly shouted to his fellow passengers and crew against the hijackers.

"Our hearts go out to those who gave their lives to stop the plane from hitting the Capitol or wherever it was going," she told AFP.

Notwithstanding a Hollywood movie, "United 93", the story of Flight 93 has largely been overshadowed by the destruction of the World Trade Center and the direct hit on the Pentagon.

In-flight recordings pulled from the rubble revealed how the passengers and crew, aware of the World Trade Center attack from mid-air cellphone calls to loved ones, fought the four hijackers for control of the Being 757.

The plane crashed at 10:03 am, slamming into the ground at 563 miles (906 kilometers) an hour, just 20 minutes' flying time from its presumed target, the Capitol building. Everyone on board died instantly.

Even without the permanent memorial, the 2,200-acre (890-hectare) site has drawn up to 6,000 people a week in the summer months, said Marlin Miller, 78, a retired Methodist pastor and volunteer guide.

But the memorial has its detractors: in the local Daily American newspaper, opponents of its design took out a large advertisement Friday to allege that its curved format resembles the Islamic crescent, facing Mecca.