President Barack Obama vowed Saturday that the United States will not waver in its fight against terrorism, as his predecessor said Americans will "never forget" 9/11 and the heroes who helped defend the country.

With the world marking a decade since the deadliest attack on US soil, and the tragedy seared into America's collective memory, communities across America honored those lost in the disaster, while New York and Washington beefed up police presence and Obama called for a "heightened state of vigilance" as Americans readied for the 10th anniversary under the shadow of a terror threat.

Relatives of victims also gathered on a field in Shanksville, Pennsylvania where former presidents George W. Bush and Bill Clinton and current Vice President Joe Biden unveiled a memorial Saturday to those who died aboard hijacked United Airlines Flight 93.

Bush honored as heroes the 40 passengers and crew who helped avert further disaster by overpowering the hijackers intent on flying the airliner into the US Capitol, saying they launched the "first counteroffensive in the war on terror."

But the president who launched that war also spoke of the wrenching pain that relatives still feel, 10 years on.

"With the distance of a decade, 9/11 can feel like a part of a different era, but for the families of the men and women stolen, some of whom join us today, that day will never feel like history," Bush said.

"The memory of that morning is fresh and so is the pain. America shares your grief," he added. "The United States will never forget."

The memorial was the most prominent yet in a series of events marking Sunday's anniversary.

New York held several events, including one in which residents somberly grasped hands in a miles-long human chain that snaked through lower Manhattan, and a memorial service for the 343 firefighters killed at the World Trade Center.

Even as US intelligence agencies chased down what officials said was a credible but unconfirmed threat of an Al-Qaeda attack around the September 11 commemorations, Obama assured terrorism would never win.

"Ten years ago, ordinary Americans showed us the true meaning of courage when they rushed up those stairwells, into those flames, into that cockpit," the president said in his radio and Internet address.

"We will protect the country we love and pass it safer, stronger and more prosperous to the next generation," he added.

"Today, America is strong and Al-Qaeda is on the path to defeat."

In London, ex-British leader Tony Blair, who played a key role in responding to the 9/11 attacks, said Western powers should be praised for reducing the terrorist threat but warned leaders not to let down their guard.

"I think we have knocked out a lot of the Al-Qaeda network," he told BBC radio, but "I don't think this is over. I think the radical Islamism which gave rise to this terrorist group is still with us."

The United States led an invasion of Afghanistan after September 11 to oust the Taliban which had been accused of harboring 9/11 mastermind Osama bin Laden, but the Islamist group insisted Saturday it had "no role whatsoever" in the attacks.

The invasion of Afghanistan "will remain a permanent stigma on the face of Western democracy," the group warned in a statement.

The specter of fresh threats hung over Americans as they readied to mark 9/11. A US official told AFP that "the general outlines of the initial report are three individuals coming into the country" last month, confirming the plot had links to militants in Pakistan.

US officials told US media that up to two of the operatives could be American citizens.

The New York Times reported that word of the plot was passed to US intelligence agents on Wednesday by an informer based in the border region between Afghanistan and Pakistan.

The informer said two US passport holders of Arab ancestry had left Afghanistan and reached the United States as recently as last week, according to the daily.

But the informer's report included only a vague physical description of the two men, the Times noted, with the first name for one given as Suliman, which is common in the Middle East.

Former national security advisor Frances Townsend told CNN Friday that US spy networks had been alerted to a new threat after intercepting communications from a known, reliable operative in Pakistan.

"It's Washington or New York. A car bomb, three men. We know that one or two are US citizens," she said.

Obama met Saturday with his national security team to review efforts to "mitigate potential terrorist threats," urging a heightened security posture even beyond September 11, the White House said.

There have been no changes to his plans to attend Sunday ceremonies at Ground Zero in New York, and Shanksville.

On Saturday Bush joined current Defense Secretary Leon Panetta and Bush-era defense chief Donald Rumsfeld at a Pentagon wreath-laying ceremony to honor the Pentagon workers and airline passengers who died there.

Memories remain raw of the day when Al-Qaeda hijackers slammed passenger planes into the World Trade Center and the Pentagon, while the fourth jet crashed in Shanksville. Almost 3,000 people were killed.

In New York, heavily armed police patrolled busy streets, trucks and cars were stopped and inspected at checkpoints and bomb-sniffing dogs scoured the subway. Similar actions were seen in Washington.