Mr Guiliani told of seeing people jumping from buildings and seeing severed body parts on the streets of Manhattan, swallowed by a "nuclear cloud" of debris.
"It was the worst experience of my life. I hope it was the worst experience I will ever have or the country will ever have," said Mr Giuliani after taking the witness stand for the prosecution.
"Every day I think about it. Every day some part of it comes back to me. I can see a person jumping, see the body parts or see a little boy or girl at a funeral," said the man who was New York's mayor at the time of the attacks.
The jury in the trial is to decide whether Moussaoui, who was in jail at the time of the attacks, should be executed for not telling authorities of the plot to blow up the World Trade Centre and other prominent buildings.
He painted a picture of 9/11 that many have yet to see as most of the graphic pictures of suicide leaps and dismembered bodies were not shown on television to avoid upsetting viewers.
Mr Giuliani was called as a witness after the prosecution told jurors that the "voices of the victims" of the attacks on New York and Washington would convince them to sentence Moussaoui to death.
They played video clips of people jumping to their deaths from the World Trade Centre and close-up images of the hijacked airliners smashing into the towers.
Jurors were also played a tape of September 11 hijacker Mohammad Atta, who mistakenly keyed the air traffic control microphone instead of the cabin intercom minutes before flying into the World Trade Centre.
"We have some planes," the accented voice of Atta could be heard saying.
"We have some planes. Stay quiet and you will be okay. We are returning to the airport."
Assistant US attorney Robert Spencer told the jury that the second phase of the sentencing trial would include human voices of the victims of September 11.
Harrowing words
Mr Giuliani, a possible Republican presidential candidate in 2008, won admiration for the way he handled the horrific aftermath of the attacks on New York.
He said the full horror struck him when he saw people leaping to their deaths from the World Trade Centre.
"It must have come from the 100th floor, way, way up," he told the court.
"I realised that I was watching this man throwing himself out. I froze. I watched him come all the way down. I realized that couple of seconds, I switched my thinking about things," he said.
"Over the course of time, I saw several people jumping. I remember seeing two people, it appeared to me as though they were holding hands. That one is probably the memory that comes back to me every day," he said.
"You could see parts of human bodies, hands and legs, a lot of injured ... this was a war, this was a battle, we were attacked. This was a battle zone," he said, keeping his composure despite his horrific evidence.
Also testifying was a former New York fireman, Anthony Sanseviro, who described how a fellow firefighter was killed by a falling body.
"I heard it coming, a whistle coming in," he said. "It just seemed like a missile coming in."
Defence lawyer Gerald Zerkin urged the jury to reject the death penalty, saying the defence would prove Moussaoui had schizophrenia, was the victim of a difficult upbringing and had been indoctrinated by al-Qaeda.
He urged jurors "not to be fooled" by Moussaoui's apparent normal behaviour in court and said several specialists would testify he is mentally ill.
As the court broke up for its morning break, Moussaoui chanted "Burn in the USA" to the melody of Bruce Springsteen's "Born in the USA."
At a later break, Moussaoui shouted "No pain, no gain, America."
The jury on Monday ruled that Moussaoui was eligible for the death penalty.
They are now being asked to weigh mitigating and aggravating factors before deciding whether the death sentence should be imposed on the man who claims to have planned to fly an airplane into the White House on September 11.