Testifying in the death penalty phase of the trial, Rudolph Giuliani painted a harrowing picture of carnage and chaos, as Manhattan streets were 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," Giuliani said after taking the witness stand for the prosecution on Thursday.
"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," the former mayor said.
It was a view of September 11, 2001 that many Americans have yet to see, as the graphic pictures of suicide leaps and dismembered bodies were mostly not shown on US television to avoid upsetting viewers.
Prosecutors called Giuliani as a witness after telling 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 twin towers, drawing gasps from the hushed courtroom.
"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"
former New York mayor
Jurors also heard the voice 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, filling the courtroom. "We have some planes. Stay quiet and you will be OK. We are returning to the airport."
In his testimony, Giuliani 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 realised 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.
Giuliani described the attacks
the worst experience of his life
"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," said Giuliani, 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."
Prosecutors called other witnesses to testify on the impact of their lives of losing loved ones.
As the court adjourned for its morning break, Moussaoui chanted "Burn in the USA" to the melody of Bruce Springsteen's song Born in the USA.
Moussaoui's defence team claim
he is mentally ill
At a later break, Moussaoui shouted "No pain, no gain, America".
Gerald Zerkin, the defence lawyer, 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.
The jury ruled on Monday that Moussaoui, who was in US custody at the time of the attacks, 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 aeroplane into the White House on September 11.