Relatives of the nearly 3,000 people who died in the September 11 attacks have called out their loved-ones names as the USA solemnly marked two decades since the attacks.

With thousands of names, it took hours to get through the list at the ceremony at Ground Zero, the site of the Twin Towers in New York, where the first of four hijacked planes crashed into the World Trade Center.

“It felt like an evil spectre 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.”

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Family members leave flowers at the National 9/11 Memorial and Museum. Photo: Getty

US President Joe Biden and former presidents Barack Obama and Bill Clinton attended the ceremony in New York while George W Bush, who was president at the time of the attacks, spoke in Pennsylvania.

In a video released on Friday night (local time), Mr 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”.

“It’s so hard. Whether it’s the first year or the 20th, children have grown up without parents and parents have suffered without children,” Mr Biden said.

The president noted the heroism that was seen in the days following the attacks.

“We also saw something all too rare: a true sense of national unity,” Biden said.

Mr 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.

US President Joe Biden (R), with former President Barack Obama (L), and former First Lady Michelle Obama at the National 9/11 Memorial. Photo: Getty

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 — Mr Bush said September 11 showed that people could 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 former Republican president.

“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 I know.”

Former US President George W. Bush at a 9/11 commemoration at the Flight 93 National Memorial in Shanksville, Pennsylvania. Photo: Getty

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

Mr Biden later commented to reporters about Mr Bush’s remarks.

“I thought President Bush made a really good speech today, a genuinely good speech, about who we are, we’re not—the core of who we are is not divided, it’s just this notion of, I don’t know how to explain it,” Mr Biden said.

Former president Donald Trump did not join the ex-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.

Julia Melendez, holding the photo of her husband Antonio, who died working at Windows on the World restaurant. Photo: Getty

In a message, former president Barack Obama said the enduring image for him and his wife Michelle of that day was not the wreckage and destruction but the people.

He singled out firefighters who ran up the stairs as others were running down, passengers who stormed the cockpit of their plane and volunteers who came forward across the country in the days that followed.

“Over the last 20 years, we’ve seen the same courage and selflessness on display again and again,” Mr Obama said.

“We saw it a decade ago when, after years of persistence, our military brought justice to Osama bin Laden. And we’re seeing it today — in the doctors and nurses, bone tired, doing what they can to save lives; the service members, some of whom weren’t even born 20 years ago, putting themselves at risk to save Americans and help refugees find a better life; the first responders battling roaring fires and rising waters to bring families to safety.”

-with AAP