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Millions of people have solemnly marked the 20th anniversary of 9/11, remembering the dead, invoking the heroes and taking stock of the aftermath just weeks after the bloody end of the Afghanistan war that was launched in response to the terror attacks.

The ceremony at ground zero in New York began exactly two decades after the deadliest act of terrorism on US soil started with the first of four hijacked planes crashing into one of the World Trade Center's twin towers.

"It felt like an evil specter had descended on our world but it was also a time when many people acted above and beyond the ordinary," said Mike Low, whose daughter Sara Low was a flight attendant on that plane.

"As we carry these 20 years forward, I find sustenance in a continuing appreciation for all of those who rose to be more than ordinary people," the father told a crowd that included US President Joe Biden and former presidents Barack Obama and Bill Clinton.

The anniversary unfolded under the pall of a pandemic and in the shadow of the US withdrawal from Afghanistan, which is now ruled by the same Taliban militant group that hosted the 9/11 plotters.

"It's hard because you hoped that this would just be a different time and a different world. But sometimes history starts to repeat itself and not in the best of ways," Thea Trinidad, who lost her father in the attacks, said before reading victims' names at the ceremony.

Bruce Springsteen and Broadway actors Kelli O'Hara and Chris Jackson sang at the commemoration but, by tradition, no politicians spoke there.

In a video released on Friday night, Biden addressed the continuing pain of loss but also spotlighted what he called the "central lesson" of September 11: "that at our most vulnerable... unity is our greatest strength".

Biden was also paying respects at the two other sites where the 9/11 conspirators crashed the jets: the Pentagon and a field near Shanksville, Pennsylvania.

Together, the attacks killed nearly 3000 people.

At the Pennsylvania site - where passengers and crew fought to regain control of a plane believed to have been targeted at the US Capitol or the White House - former president George W Bush said September 11 showed that people can come together in the US despite their differences.

"So much of our politics has become a naked appeal to anger, fear and resentment," said the president who was in office on 9/11.

"On America's day of trial and grief, I saw millions of people instinctively grab their neighbour's hand and rally to the cause of one another. That is the America know."

"It is the truest version of ourselves. It is what we have been and what we can be again."

Calvin Wilson said a polarised country has "missed the message" of the heroism of the flight's passengers and crew, which included his brother-in-law LeRoy Homer.

"We don't focus on the damage. We don't focus on the hate. We don't focus on retaliation. We don't focus on revenge," Wilson said before the ceremony.

"We focus on the good that all of our loved ones have done."

Former president Donald Trump did not join the two other past presidents at the anniversary ceremonies but visited a fire station and police precinct in New York, where he criticised his successor for the way US forces pulled out of Afghanistan last month.

Other observances - from a wreath-laying in Portland, Maine, to a fire engine parade in Guam - were planned across a country now full of 9/11 plaques, statues and commemorative gardens.

The attacks ushered in a new era of fear, war, patriotism and, eventually, polarisation.

They redefined security, changing airport checkpoints, police practises and the government's surveillance powers.

A "war on terror" led to invasions of Iraq and Afghanistan, where the longest US war ended last month with a hasty airlift punctuated by a suicide bombing that killed 169 Afghans and 13 US service members and was attributed to a branch of the Islamic State extremist group.

© AP 2021

IMAGE CREDIT: Michael Foran, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons