"Our enemy is not each other but the terrorists who attacked us," Mr Giuliani said in his opening statement to the panel on the hearing's second day in Manhattan. "The blame should clearly be directed at one source and one source alone: the terrorists who killed our loved ones," he continued, as family members of the victims broke into applause.
But his testimony was later interrupted by outbursts from victims' families which resulted in one man being removed from the room, just 1.5 miles from Ground Zero, after he shouted at the panel to "ask some real questions".
Mr Giuliani appeared to be trying to deflect criticism from his former police and fire chiefs, who were widely hailed after the attacks but harshly criticised on Tuesday for inter-departmental rivalry which contributed to them failing to communicate effectively on September 11. Commissioner John Lehman called the lapses a scandal "not worthy of the Boy Scouts, let alone this great city".
Ex-fire commissioner Thomas Von Essen called Mr Lehman's comments "despicable". The former mayor acknowledged there were "terrible mistakes" but in unprecedented circumstances.
Mr Giuliani said he was not warned of a possible terrorist attack on New York City contained in an August 2001 White House briefing paper to President Bush, which suggested terrorists were casing buildings in New York.
But he believed such information would have made little impact on local security precautions even though it mentioned the World Trade Centre three times.
"If that information had been given to us, or more warnings had been given in the summer of 2001, I can't honestly tell you we'd do anything differently," Mr Giuliani testified. "We were doing at the time everything we could think of ... to protect the city."