WATCH: World marks 15 years since 9/11 terror attacks

Nearly 3,000 people were killed after four planes were hijacked

 WATCH: World marks 15 years since 9/11 terror attacks

Smoke pours from one of the towers of the World Trade Centre on Tuesday, September 11, 2001 | Image: CHAO SOI CHEONG / AP/Press Association Images

This weekend marks 15 years since the 9/11 terror attacks on the United States.

On September 11th 2001, 19 terrorists hijacked four commercial airplanes in a coordinated attack.

They flew two of the planes into the Twin Towers of the World Trade Centre in New York, and a third into the Pentagon.

Learning about the other hijackings, passengers and crew on the fourth plane launched a counterattack, forcing the hijacker pilot to crash the plane into a field in Pennsylvania.

Nearly 3,000 people were killed - the single largest loss of life from a foreign attack on American soil.

People flee the scene near New York's World Trade Centre after terrorists crashed two planes into the towers Tuesday, September. 11, 2001 | Image: DIANE BONDAREFF / AP/Press Association Images

A ceremony will be held on Sunday at the site where the Twin Towers stood, beginning at 8:46am local time - the time the first plane hit.

Similar commemorations will also be held around the world.

The site in New York is now the 9/11 Memorial, taking up eight of the 16 acres at the World Trade Centre.

The names of every person who died in the attacks are inscribed in bronze around twin memorial pools.

The anniversary comes just days after an American flag, raised above the rubble on 9/11, went on display at the Memorial museum.

The flag, which three New York City firefighters raised above World Trade Centre, disappeared after the attack.

But with the help of the flag's original owner Shirley Dreifus - in honour of her late husband Spiros E Kopelakis - the flag was donated.

The Ground Zero flag on view in the 9/11 Memorial Museum | Image: Jin Lee

Its authenticity was determined through a months-long forensic investigation.