This fifth moment of silence commemorates a fourth plane, Flight 93, which crashed into a field in Shanksville, Pennsylvania, at 10:03 local time (15:03 BST).

It is thought the hijackers had meant to attack the US Capitol building in Washington DC - but passengers on the plane fought back.

This next moment of silence marks the collapse of the World Trade Center's South Tower at 09:59 local time (14:59 BST).

There will be two more this morning.

Secunder Kermani
BBC News, Kabul

Image source, Secunder Kermani

At a busy market in the west of Kabul, hardly anyone was aware that today marks the 20th anniversary of the 9/11 attacks.

Some were completely unaware of what had happened in New York two decades ago.

For others, their memory of the tragedy is steeped with Afghanistan’s own struggles.

One man told us he did feel sympathy for those killed, but added, "we have suffered more than 9/11 in Afghanistan".

Another, born just a few months after the 11 September attacks was more positive about America’s legacy in the country. "They have changed our lives," he said, citing improvements to female education.

Now, he was deeply anxious and was looking for a way out of the country.

Two decades after they were overthrown, the Taliban’s flags flutter above the market and across Kabul once again.

Many residents are bitter are about the past, but are more focused on the present.

One elderly woman told us: "What I want is peace and security, an end to inflation and jobs for young people."

Laura Trevelyan
BBC World News America presenter

Image source, EPA

It’s notable how many children born after 9/11 are reading the names of those killed in the attacks at this moving ceremony here at the site of the World Trade Center.

An entire generation has been born after the terror attacks.

And now, young relatives of those who died are taking part in the solemn commemoration, ensuring that what happened here is remembered by those too young to have witnessed the fateful day.

Another moment of silence has begun, marking the moment that a third plane struck the Pentagon, at 09:37 local time (14:37 BST).

The attack killed 125 people at the US military headquarters and destroyed part of the iconic building's western side.

Dan Thomas
BBC News, Ground Zero, New York City

Travelling to the World Trade Center from Midtown it feels like it could be any other September day in New York. Early morning workers take the subway. The sky is clear blue and the sun is shining.

But on Church Street near the World Trade Center, the mood is more tense. There’s a heavy security presence and camera crews throng the area. Members of the public make their way to the memorial site carrying wreaths.

Uniformed members of the military, fire and police departments from across the US are here for the memorial. Many attend every year and there’s a sense that this is as much a reunion as it is a commemoration.

Gilbert is a member of the New York City Fire Department who lost friends in the tragedy.

"You just can’t believe it’s been 20 years, it seems like yesterday," he says.

He says it doesn’t get any easier over the years and every September "brings everything back".

Attending the memorial helps, he says, and the fact firemen from around the world attend to show their respect "means a lot".

"But it still hurts," he says.

Let's return again to the quiet crowd in New York City, listening intently to this poignant roll call.

The family members tasked with reading the long list of names take pauses to remember their own loved ones.

One woman remembers her brother.

"I look at your nephew's face and I see you," she says.

"So much has changed in our lives, but the memory of you... never changes and never will," she adds.

Another woman fights back tears as she speaks of her husband, holding up a photo of a man in a tuxedo.

"Continue to watch over us and your family," she says, her voice breaking.

"Until we meet again my love."

Gary O'Donoghue
BBC News Correspondent, Pentagon

Image source, Getty Images

Once again, as dawn broke over the Pentagon, the attack on the symbol of American military might was marked in an atmosphere of quiet reflection.

There was, as ritual has come to dictate, the unfurling of the stars and stripes on the western face of the building at the point Flight 77 struck 20 years ago.

Two pipers played Amazing Grace as a small group of military leaders looked on in solemnity, the building bathed in blue light.

That western facade of this iconic building was rebuilt within a year of America's worst-ever terrorist attack. A chapel of remembrance now marks the spot where the hijackers aimed their weapon.

Despite the tragedy, the building never stopped functioning; the US was under attack, and the men and women of the world's most powerful military immediately set about planning the country's response.

Twenty years on, few would have imagined that American soldiers would spend all that time in Afghanistan, and even fewer would have imagined their departure would be so chaotic.

Image source, Getty Images

Wearing a black suit and tie, Bruce Springsteen takes the stage in New York.

The New Jersey native, born just across the Hudson River from Manhattan, sings his 2020 song, "I'll see you in my dreams".

"The road is long and seeming without end," he sings. "The days go on, I remember you my friend."

The crowd is quiet, some holding each other and swaying slightly.

Image source, Getty Images

Family members are reading the names of their loved ones lost.

Many are speaking through tears, voices shaky. These are mothers, fathers, wives, husbands, sons and daughters, nieces and nephews.

Each also offers a few words in memory.

"Our son is the spitting image of you," one woman says. "Twenty years feels like an eternity, but it still feels like yesterday."

Nearly 3,000 names will be read over the course of today, honouring the 2,983 victims of the 9/11 attacks on the World Trade Center, the Pentagon and Flight 93, as well as the six people killed in the 1993 World Trade Center bombing.

Another moment of silence is being held, marking the moment the World Trade Center's South Tower was struck, at 09:03 local time (14:03 BST).

As the reading of the names goes on, let's recall some of the experiences of that day. When a plane crashed into the World Trade Center's North Tower, Tom Canavan was on the 47th floor. He told the BBC what happened that day.

"Me and my colleagues got down the stairs and we were going through the ground level. I turned back to help an elderly couple through the doors.

"Then the South Tower collapsed and rubble fell on top of us. It killed my co-workers who were walking ahead of me.

"I’m not sure if I was knocked unconscious, but at some point, I was able to dig myself out and escape outside. I was lucky to get off the plaza alive, because of what was hitting us from the sky. When you see the footage of that cloud on TV, you don’t see what’s in that dust, but actually, the smoke was full of copy machines, computers, telephones, chairs - and people.

"I was very, very lucky. I got half a block away before the North Tower came down too and I was hit again by the debris.

"I was taken to hospital and I was able to speak to my wife on the telephone. She was pregnant with our daughter, and my son was turning three years old on 17 September.

"That day, split-second decisions determined if you lived or died. I think a lot about other people who made a decision or just, by fate, made a left instead of a right and didn’t make it out."

Image source, National Geographic

Image source, Getty Images

Family members are reading the names of their loved ones out loud. It's a very powerful moment and some are finding it hard to compose themselves.

Mike Low, who lost his daughter Sarah on 9/11, introduced the reading.

Speaking slowly, solemnly, he thanks those who helped him and his family, who "pulled us through the darkest days of our lives".

He remembered the "grey and black world" of New York on that day, and remarks on the time since, "years we have filled speaking out on my daughter's behalf".

He asks for the history of 9/11 to be remembered, "not as numbers or a date, but the faces of ordinary people".

This is to mark the time the first of the Twin Towers was struck that morning, at 08:46 local time (13:46 BST).

The names of victims wlil be read aloud shortly.

President Joe Biden and First Lady Jill Biden have just arrived at the 9/11 memorial in New York City.

He is joined by former presidents Barack Obama and Bill Clinton, as well as several national and state leaders.

The service will begin within minutes.

Jon Sopel
BBC North America Editor

Within weeks of the attack, a massive multinational force was assembled, with America spearheading, to force the Taliban - who'd allowed the al-Qaeda leaders who plotted the 9/11 attacks to set up terrorism finishing schools in the country - out of power.

This would be the start of a series of American entanglements. And one thing they all seemed to share is a lack of long term success.

The "liberal interventionism" championed by both President Bush and Tony Blair saw the Taliban booted out of Afghanistan, and Saddam Hussain ousted in Iraq - but what followed became known as the "forever wars".

When President Obama came into power there was a different approach.

In Libya the Americans would support the overthrow of Colonel Gaddafi, but at arm's length; with the Syrian civil war, America huffed and puffed but chose to sit that one out. In Egypt, the US supported the overthrow of President Mubarak - but when the Egyptians elected the Muslim Brotherhood to power, the Americans made clear their displeasure, and the generals moved back in.

In other words, the Americans had tried invasion - it went badly. They'd tried "intervention-lite" - still no good. Staying outside a conflict brought criticism. It's a pretty unhappy tale of foreign intervention.

The reasons for Donald Trump's victory in 2016 are various. But one of them was certainly the growing weariness to the cost of these military entanglements.

Twenty years ago, Americans in the face of tragedy were united. Today, with relative peace and considerable prosperity they are bitterly divided.

Read more here.

Image source, Getty Images

John Simpson
World Affairs Editor

The Khyber Pass is a grim place, marked with the signs of past conflict. At Torkham, where Pakistan borders Afghanistan, there’s tension and fear as hundreds of Afghans gather in the hope of getting across. The Pakistani soldiers square off against Taliban guards, burly men with black beards and turbans. Overhead the white Taliban flag with the Shahada or statement of Muslim faith written on it, flies above the border crossing - not the old national Afghan one.

The only people who are allowed through are those with documents showing they need medical attention in Pakistan, plus their family members. In the heat, the other would-be emigrants crowd together behind the wire, looking on enviously.

The Taliban guards were mostly happy to talk - though one I spoke to was given a dressing-down by his commander and marched off. To me, having spent a good deal of time with the Taliban before and after 9/11, the rank-and-file seemed exactly what they’ve always been: backwoodsmen, with totally unreconstructed views.

It was hard to remember that the Taliban soldiers I came across at Torkham were an entire generation younger than the ones I met in Taliban-occupied Afghanistan from 1996 to 2001. Nothing about them seemed to have changed or developed. Certainly not their ideas and attitudes.