Friday's milestone anniversary has brought memorial rituals - wreath-laying, flag-flying, moments of silence, vigils and readings of victims' names - in tribute to a day that about half of US adults remember as the most historic event of their lifetime, according to a new poll.
President Donald Trump marked the anniversary by attending a ceremony at the Pentagon commemorating the lives lost when al-Qaeda militants hijacked four planes and killed nearly 3000 people in three locations.
Trump and first lady Melania Trump listened as the names of the 184 people who died at the Pentagon that day were read aloud at the start of the ceremony.
The president began his remarks by recalling American unity following the attacks.
"In those harrowing days after the attacks, we saw the authentic spirit of our nation," Trump said at the Pentagon.
"We were all New Yorkers that day. Every one of us. We were all New Yorkers. We were all family.
"September 11th reminded us of the terrible power of hate. The 25 years since have shown us the even greater power of the American spirit to endure and prevail, and to win."
In a downpour at daybreak, Defence Secretary Pete Hegseth and top military officials stood outside and watched as a giant American flag was unfurled on the building struck by one of those planes a quarter of a century ago.
George W. Bush, who was in office on 9/11, attended the observance at the National September 11 Memorial in New York, along with former presidents Bill Clinton, Barack Obama and Joe Biden, and current Vice President JD Vance.
Commerce Secretary Howard Lutnick wiped away a tear as he revisited the place where the attacks killed 658 of his colleagues - including his brother, Gary Lutnick - at the investment firm Cantor Fitzgerald.
Other administration officials were at a ceremony in Shanksville, Pennsylvania, where a fourth plane crashed after passengers and crew members stormed the cockpit and fought the hijackers.
Amid the post-9/11 appeals for national unity, the attacks also fanned mistrust, prejudice and hate crimes directed at Muslims.
In the quarter of a century since, Muslim Americans have played a growing role in public life from statehouses to Congress to New York City Hall, where Zohran Mamdani is the city's first Muslim mayor.
He faced criticism from some for attending the anniversary ceremony at ground zero.
Republican former governor George Pataki and 9/11 victims' relatives cited, among other things, past remarks that some of Mamdani's political associates have made about the attacks.
Former mayor Rudy Giuliani also opposed Mamdani's attendance, though the two shook hands on Friday.
Mamdani said this week he loves the city and country and that he wanted the anniversary to focus on victims' families, first responders and New Yorkers who helped one another after the attacks, which he called a "horrific act of terror".
The 34-year-old mayor was an elementary school student in New York on 9/11.
One victim's son and namesake, Michael Massaroli, used his time at the podium on Friday to deplore people "using this tragedy to spread hate" against Muslims and others.
"That's wrong, and that does no honour to the victims here," he said.
