Twenty-five years after 9/11, US presidents past and present have commemorated the lives lost when hijacked planes slammed into the World Trade Center, the Pentagon and a Pennsylvania field.

President Donald Trump led a Pentagon ceremony, while US Vice-President JD Vance attended a memorial at the site of the World Trade Center in New York, along with all four living former US presidents and first ladies.

Moments of silence were held to mark the timings of the attacks and the names of nearly 3,000 people who died were read out.

Two beams of light illuminated the Manhattan sky for a Tribute in Light on Friday evening, as other landmarks in the state lit up in blue in commemoration.

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George W Bush, who was president at the time of the attacks, was joined by former Presidents Joe Biden, Barack Obama and Bill Clinton at Friday's ceremony at the World Trade Center site.

At the Pentagon, President Trump attended a commemoration alongside cabinet secretaries and Condoleezza Rice, who was national security adviser on 9/11.

"Today we renew the sacred oath that we first swore in their names a quarter of a century ago," Trump said. "We will never, ever forget. That's why we fight today."

He sought to link the "war on terror" that followed 9/11, which sparked years of US involvement in Middle East wars, with his current conflict in Iran.

Trump told the crowd that the Islamic Republic "will never, ever have a nuclear weapon".

Commemorations were also held in Shanksville, Pennsylvania, where Flight 93 crashed after passengers fought back against the hijackers.

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Janice Brooks had only moved to New York a couple of weeks before 9/11 to begin what she calls "the biggest adventure", working for a company on the 84th floor of the South Tower of the World Trade Center.

She told the BBC she was in the office when the first plane hit the North Tower. While she was evacuating, the second plane hit with an "almighty thud".

Brooks said she heard "one of those horror film screams" as a woman was hit by debris. She looked up and saw a "big gaping hole" where her office should have been.

Every year on 11 September, bells ring to commemorate the exact time that each of the four planes crashed, and the exact time the Twin Towers fell.

Each of the six bells is followed by a moment of silence.

This year, a seventh bell was rung to commemorate the thousands who died from diseases including cancer after the attacks.

Illnesses related to dust and smoke from the towers' collapse have since claimed more than threefold the nearly 3,000 lives lost in the attacks, US health officials say.

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As the names of the dead were read out for nearly four hours at the ceremony in Manhattan, a 9/11 widow criticised Saudi Arabia and what she characterised as failures by US officials to hold the kingdom accountable.

"You are missed more with each year that passes," Terry Strada said of her husband Tom Strada in remarks broadcast live worldwide.

"But how can it be 25 years and still we have no justice for the role that Saudi Arabia played in your murder?"

• You can listen to Terry Strada's story in the BBC radio documentary Intrigue: A 9/11 story

The comments by Strada, national chairperson of 9/11 Families United, drew applause from the audience.

The majority of those who carried out the 9/11 attacks were from Saudi Arabia and planning was led by al-Qaeda founder Osama Bin-Laden, scion of a prominent Saudi family.

But Saudi officials have repeatedly denied any involvement in the attacks, and in 2004 the 9/11 Commission Report "found no evidence that the Saudi government as an institution or senior Saudi officials individually funded" al-Qaeda.

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Events were also held elsewhere to honour the victims. In London, there was a ceremonial event at Buckingham Palace attended by Foreign Secretary Ed Miliband and the US ambassador Warren Stephens.

The Toronto Stock Exchange also held an event to remember colleagues who died in the attacks.

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In Gander, Newfoundland, the US ambassador to Canada praised the small town that took in thousands of passengers from 38 diverted planes in the hours after the attack.

"When people in America hear the word Gander, our hearts soften," Pete Hoekstra said.

"As a representative of the United States government, thank you for your friendship... to the nation of Canada, thank you for being a partner and for the shared sacrifices that you made with us."

His remarks come amid fraught relations between the US and Canada, with the two countries engaged in a trade war. Ahead of the 9/11 commemorations, some Gander residents had called for Hoekstra's invite to be rescinded.

Still, Hoekstra's speech was met with applause. But in separate remarks afterwards, former Canadian Prime Minister Jean Chrétien said: "I have message for American friends... never mistake our generosity for weakness." He received a standing ovation.

In Milan, a group of Italian and US firefighters participated in a ceremonial stair climb to honour the emergency responders who died in the line of duty.

Dressed in their uniforms and carrying equipment, they climbed 110 floors up and down the Galfa Tower, in remembrance of the number of storeys on each of the Twin Towers.

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