Blair, making a second appearance before the panel to clarify evidence he gave them a year ago, also urged Western leaders to confront a growing threat posed by Iran.
Addressing the five-member panel scrutinizing Britain's role in the unpopular war, Blair, 57, acknowledged that in phone calls and messages in 2002 - months before Parliament approved Britain's role in the conflict - he reassured President George W. Bush and told him: "You can count on us."
Alongside his evidence, the inquiry published a previously unseen 2002 memo from Blair to his chief of staff, in which the leader called for a "gung-ho" approach toward Saddam Hussein's regime.
Critics of the war hope the inquiry will conclude Blair had been determined to back the U.S. invasion, whether or not it was supported by the public, Parliament or legal opinion.
Following his initial hearing, Blair was sharply criticized for suggesting he had no regrets over the decision to join the 2003 U.S.-led invasion.
"I want to make it clear that of course I regret deeply and profoundly the loss of life," he said Friday, his voice faltering with apparent emotion, "whether from our own armed forces, those of other nations, the civilians who helped people in Iraq or the Iraqis themselves."
Audience members shouted: "Too late, too late," while two women turned their backs on Blair and then walked out.
"Your lies killed my son, I hope you can live with yourself," Rose Gentle, whose 19-year-old son Gordon Gentle was killed while serving in Basra, southern Iraq, in 2006, shouted as Blair completed about four hours of testimony.
A note prepared by a senior adviser in December 2001 - and published Friday - warned Blair that the legal case for military action would be "threadbare." In the newly published March 2002 memo to his chief of staff, Blair - aware that the United States was pushing the case for regime change - said Britain "should be gung-ho on Saddam." But he acknowledged it would be difficult to convince skeptics, and said that Iraq's weapons program didn't "seem obviously worse than three years ago."
"People believe we are only doing it to support the U.S., and they are only doing it to settle an old score," he wrote.
Under questioning, Blair angrily denied the decision to invade Iraq had emboldened neighboring Iran, or encouraged Tehran to press ahead with its attempts to develop nuclear weapons.
"This is a looming and coming challenge," Blair said, calling for decisive action on Iran.