Former New York Mayor Rudolph Giuliani has described seeing the horror of the 9/11 attacks on the city's twin towers.
Moussaoui listened to Mr Giuliani give evidence for the prosecution
He told the trial of Zacarias Moussaoui he had been unwilling to believe people were jumping from the buildings until he had seen it with his own eyes.
Mr Giuliani testified for the prosecution, along with relatives of some of those killed in the attacks.
The jury has to decide whether the self-confessed al-Qaeda member Moussaoui should be executed.
On Monday, jurors said Moussaoui could face the death penalty. He is the only man to be charged in the US in connection with the 9/11 attacks.
The 37-year-old French citizen has pleaded guilty to six counts of conspiracy to attack the US.
Mr Giuliani told the jury the image of two people jumping together from the World Trade Center, holding hands, remained with him every day.
Describing the moment he saw people falling, he said: "I froze. I realised that in a couple of seconds, it switched my thinking and emotions. I said 'We're in uncharted territory'."
Rudolph Giuliani (left) said the twin towers were a "symbol" of New York
The human and emotional costs of the attacks, when hijacked planes were deliberately flown into New York's twin towers and the Pentagon, are taking centre stage at the trial.
During Mr Giuliani's testimony, the jury was played video clips of the planes crashing into the twin towers, and people jumping from the buildings.
The former mayor, who was sitting next to scale models of the World Trade Center in court, said: "By the time the second plane hit, we knew for sure it was a terrorist attack."
He added that after the towers collapsed, it looked like a "nuclear cloud" was going through Manhattan, Reuters news agency reported.
The scene at the World Trade Center site was "horrid".
"It was the worst thing I have ever seen in my life," Mr Giuliani told the jury.
"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," the AFP news agency quoted him as saying.
A former New York fireman, Anthony Sanseviro, also testified on Thursday, describing how a colleague had been killed by a falling body, AFP reported.
"I heard it coming, a whistle coming in," he told the court. "It just seemed like a missile coming in," he said.
Jurors also heard from relatives of some of the 9/11 victims, who described the impact the attacks had had on their lives.
Prosecutor Robert Spencer told the jury that what it would hear from families of the victims would "be all you need in this case to sentence the defendant to death".
But defence lawyer Gerald Zerkin urged the jury to remain open to "the possibility of a sentence other than death", according to the Associated Press agency.
He said the defence would prove Moussaoui suffered from mental illness and had had a difficult upbringing, and urged jurors "not to be fooled" by his client's apparently normal behaviour in court.