Americans are marking 24 years since the September 11, 2001, attacks with solemn ceremonies, volunteer work and other tributes honouring the victims.

Many loved ones of the nearly 3,000 people killed will join dignitaries and politicians at commemorations on Thursday in New York, at the Pentagon and in Shanksville, Pennsylvania.

Others choose to mark the day at more intimate gatherings.

James Lynch, who lost his father, Robert Lynch, during the World Trade Center attack, said he and his family will attend a ceremony near their hometown in New Jersey before spending the day at the beach.

“It’s one of those things where any kind of grief, I don’t think it ever goes away,” Lynch said as he, his partner and his mother joined thousands of volunteers preparing meals for the needy at a 9/11 charity event in Manhattan the day before the anniversary. “Finding the joy in that grief, I think, has been a huge part of my growth with this.”

The remembrances are being held during a time of increased political tensions. The 9/11 anniversary, often promoted as a day of national unity, comes a day after conservative activist Charlie Kirk was shot and killed while speaking at a college in Utah.

Kirk’s killing is expected to prompt additional security measures around the 9/11 anniversary ceremony at the World Trade Center site in New York, authorities said.

At ground zero in lower Manhattan, the names of the attack victims will be read aloud by family and loved ones in a ceremony attended by Vice President JD Vance and his wife, second lady Usha Vance. Moments of silence will mark the exact times when hijacked planes struck the World Trade Center's iconic twin towers, as well as when the skyscrapers fell.

At the Pentagon in Virginia, the 184 service members and civilians killed when hijackers steered a jetliner into the headquarters of the US military will be honoured. President Donald Trump and first lady Melania Trump will attend the service before heading to the Bronx for a baseball game between the New York Yankees and Detroit Tigers Thursday evening.

And in a rural field near Shanksville, Pennsylvania, a similar ceremony marked by moments of silence, the reading of names and the laying of wreaths, will honour the victims of Flight 93, the hijacked plane that crashed after crew members and passengers tried to storm the cockpit. That service will be attended by Veterans Affairs Secretary Doug Collins.

Like Lynch, people across the country are also marking the 9/11 anniversary with service projects and charity works as part of a national day of service. Volunteers will be taking part in food and clothing drives, park and neighborhood cleanups, blood banks and other community events.

In all, the attacks by al-Qaeda militants killed 2,977 people, including many financial workers at the World Trade Center and firefighters and police officers who had rushed to the burning buildings trying to save lives.

The attacks reverberated globally and altered the course of US policy, both domestically and overseas. It led to the “ Global War on Terrorism ” and the US-led invasions of Afghanistan and Iraq and related conflicts that killed hundreds of thousands of troops and civilians. (AP)