THE 343 firefighters who died 15 years ago in the September 11 attacks have been honoured at a St Patrick’s Cathedral memorial service for acts of heroism that New York’s mayor said “most of us could not fathom.”

Hundreds of family members of those who died and those who survived the attacks listened to their loved ones remembered for their courage on a day that would claim the lives of more firefighters than any other day in the nation’s history.

Each of the names of the dead firefighters was read aloud during the service.

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Attorney General Loretta Lynch read a letter from President Barack Obama that described the effect left on the nation after hijacked planes struck the twin 110-story towers of the World Trade Centre, causing their collapse.

“We remember that no act of terror can match the character of our country, change who we are as Americans or overcome our enduring resolve to be each other’s keepers in times of calm and crisis alike,” the letter said.

In a service presided over by Cardinal Timothy Dolan, Mayor Bill de Blasio noted that the firefighters killed ranged in age from 22 to 71.

He said everyone in New York that day suffered, but none more than members of the Fire Department of New York.

“We remember what the members of the FDNY did in the face of something that could not be imagined. They showed selflessness. They showed courage,” he said.

“When we needed them most they showed us the best, the best in humanity.”

De Blasio said the firefighters “did things that day that most of us could not fathom. They rushed to danger, not from it.”

The mayor also noted that some firefighters died after 9/11 from diseases linked to the contaminated air they were exposed to that day and that others are still alive but suffering.

The massive Catholic cathedral was filled to capacity for the afternoon service with city dignitaries, firefighters and families of those killed.

A procession into the cathedral featured a banner with the number 343, followed by firefighters carrying American flags representing every member of the department killed that day.

The main memorial service will be held at the National September 11 Memorial and Museum in New York, where the names of victims will be read starting at 8.46am local time, which marks the time the first hijacked aeroplane hit the North Tower of the World Trade Centre.

Extremists on suicide missions carried out the 2001 attacks, hijacking four passenger jets and crashing them into the two World Trade Center towers in New York and the Pentagon defence headquarters outside Washington.

Services will also be held in Washington and a memorial site just outside of Shanksville, Pennsylvania, where the fourth plane crashed after its passengers apparently rose up against the hijackers and crashed it in a field, thwarting an attack on Washington.

The attacks killed 2977 people.