A noontime ceremony is planned to honor the six people who died in the 1993 blast in an underground garage below one of the twin towers. More than 1,000 people were injured.
The attack was the first dramatic demonstration that "terrorism is theater and New York is the biggest stage," said Police Commissioner Raymond Kelly.
The ceremony was being held at the 9/11 memorial that honors more than 2,700 people who died in the 2001 terrorist attack at the World Trade Center.
A moment of silence will be observed at 12:18 p.m., the time when a truck bomb was detonated below the north tower.
Six Islamic extremists were convicted of carrying out the bombing, including mastermind Ramzi Yousef.
At first, officials assumed the explosion on a chilly day was an accident.
The initial report to police that day called it an apparent transformer explosion at the trade center.
Kelly raced to the scene, where the bomb planted in a parked Ryder van had left a crater half the size of a football field in the trade center garage, causing more than a half-billion dollars in damage.
"I remember seeing this tremendous sea of first-responder vehicles ... and smoke was coming out," said Kelly, who was on his first stint as police commissioner.
The trade center stood in the darkness that night for the first time since it opened in 1973.
It was only the next day, after a utility mishap was ruled out, that authorities "started to come to the conclusion it was bomb," Kelly said.
Investigators then found a vehicle identification number on a piece of the blown-up van that they traced to Mohammed Salameh, who had rented the van.
