The United States commemorated the 25th anniversary of the devastating September 11, 2001 attacks Friday with a poignant ceremony in New York marked by President Donald Trump's absence and searing criticism from victims' relatives.

In an unusual move, Trump paid his respects at the Pentagon, not the event in New York, where hijacked planes destroyed the World Trade Center and most of the 2,977 victims were killed, upending the global status quo.

The Republican billionaire 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.

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

"We don't have a choice. There can only be victory. We fight hard. We fight to win."

Vice President JD Vance and Trump's four living predecessors -- Republican George W. Bush, who was in office during the attacks, and Democrats Joe Biden, Barack Obama and Bill Clinton -- attended the event at Ground Zero in lower Manhattan. 

There, a huge crowd gathered for the somber annual ceremony, which began just after 8:40 am (1240 GMT) and retraces the events of 9/11 minute by minute.

As usual, the names of the victims were read out, and a bell was rung to mark major moments in the attack.

The event is generally apolitical with no speeches.

But several bereaved family members who read out the names of those killed criticized Saudi Arabia and past US leaders' closeness to the kingdom.

'We have no justice'

Terry Strada, whose husband Tom Strada died in the Twin Towers, unleashed a stinging attack on Riyadh, accusing it of complicity in the attacks, and denouncing former presidents who were in attendance.

"You are missed more with each year that passes, but how can it be 25 years and still we have no justice for the role that Saudi Arabia played in your murder?" she said in remarks broadcast live worldwide.

The September 11, 2001 attacks were carried out by mostly Saudi hijackers orchestrated by mastermind Osama bin Laden, a wealthy Saudi.

"For 25 years, administration after administration -- including leaders in front of us here today -- chose to protect the Saudis instead of standing with a 9/11 family," Strada said, calling for action from the absent Trump.

In New York, Democratic Mayor Zohran Mamdani, the Big Apple's first Muslim leader, attended the Ground Zero event despite a campaign urging him to stay away.

That effort was spearheaded by the right-wing New York Post newspaper and former mayor and Trump ally Rudy Giuliani, who led the city through the crisis and was also present on Friday.

The petition, which has gathered more than 100,000 signatures, accuses Mamdani of associating with unspecified individuals who are "dismissive of American institutions or insufficiently critical of extremist ideologies."

Mamdani won plaudits for recently revealing thousands of documents showing what city officials knew about toxicity levels of the air in Lower Manhattan following the attacks.

After the initial dust cloud that followed the collapse of the World Trade Center, months of pollution and lingering dust ensued, causing cancers from which thousands of New Yorkers still suffer today.

'It's up to us'

Nineteen Al-Qaeda operatives hijacked four airliners on the nearly cloudless autumn day in 2001. 

Two of the jets slammed into the World Trade Center's Twin Towers, causing the iconic skyscrapers to collapse.

A third plane struck the Pentagon, while a fourth -- believed to have been aiming for Congress or the White House -- crashed into a field near Shanksville, Pennsylvania, after passengers fought back.

Some 100 million Americans have been born since or are too young to remember the traumatic day.

Retired New York firefighter John Fila said his firehouse lost 15 members on 9/11.

"In our firehouse there's the picture of the 15 that we lost. (New firefighters) see that but it's up to us... to explain those firefighters' lives and what they were doing that day," he said.

Al-Qaeda meanwhile marked the anniversary by releasing a video showing images of the late Bin Laden and his son Hamza, who Trump said in 2019 had also been killed.