Tony Blair has said he "regrets deeply and profoundly the loss of life" during and after the 2003 Iraq war.
The ex-PM said his refusal to express regret for the decisions that led to war at his first appearance before the committee had been misinterpreted.
But his words were met with cries of "too late" from the public gallery.
Mr Blair also urged the West to stop apologising for its actions and warned of the threat from Iran, during a four-hour grilling by the inquiry.
Asked whether what had happened in Iraq had made the risk from Iran and other countries developing nuclear weapons worse, rather than better, he said: "I don't think so."
Mr Blair, who is now a UN Middle East peace envoy, said there was "a looming and coming challenge" from Iran.
"I am out in that region the whole time. I see the impact and influence of Iran everywhere. It is negative, destabilising and it is supportive of terrorist groups," Mr Blair told the inquiry.
He said Iran "is doing everything it can to impede progress in the Middle East peace process, and to facilitate a situation in which that region cannot embark on a process of modernisation it so urgently needs".
He added: "And this is not because we have done something. At some point - and I say this to you with all the passion I possibly can - the West has got to get out of what I think is this wretched policy, or posture, of apology for believing that we are causing what the Iranians are doing, or what these extremists are doing. The fact is we are not.
"The fact is they are doing it because they disagree fundamentally with our way of life and they will carry on doing it unless they are met with the requisite determination and, if necessary, force."
In a personal statement at the end of his evidence session, Mr Blair said it was never his "meaning or intention" to say he had no regrets about the loss of life in Iraq when he appeared before the Iraq inquiry last January.
"I wanted to make that clear, that of course, I regret deeply and profoundly the loss of life, whether from our own armed forces, those of other nations, the civilians who helped people in Iraq, or the Iraqis themselves and I just wanted to say that because it is right to say that and it is what I feel."
Committee chairman Sir John Chilcot had to tell the public gallery to be quiet as some members shouted "too late".
Several people walked out and Rose Gentle, whose son was killed in Iraq, told the former prime minister that she did not believe him, adding: "I hope you can live with it."
Earlier, Mr Blair revealed that he had privately assured US President George Bush "you can count on us" eight months before the invasion.
He also revealed he disregarded Lord Goldsmith's warning that attacking Iraq would be illegal without further UN backing because the advice was "provisional".
The ex-PM said he had believed his top legal officer would change his position on whether a second UN resolution justifying force was needed when he knew the full details of the negotiations.
Sir John repeated his call for the private statements Mr Blair made to Mr Bush and then US defence secretary Donald Rumsfeld in July 2002, to be made public, saying the panel was "disappointed" that this had not happened.
The panel have seen the notes but they will remain secret after Cabinet Secretary Gus O'Donnell said releasing them would compromise diplomatic confidentiality.
Mr Blair said that, although he agreed with Sir Gus's decision, he was "not going to hide behind the cabinet secretary".
Summing up the contents of the statements, he said he had told Mr Bush: "You can count on us, we are going to be with you in tackling this, but here are the difficulties."
The message he wanted to get across, he added, was "whatever the political heat, if I think this is the right thing to do I am going to be with you, I am not going to back out if the going gets tough. On the other hand, here are the difficulties and the UN route is the right way to go".
'Tapestry of deceit'
Mr Blair was also quizzed about apparent discrepancies between what he told the committee in January 2010 and recent statements to the committee by his Attorney General Lord Goldsmith.
Lord Goldsmith said he had been "uncomfortable" with statements Mr Blair made in the Commons ahead of the war suggesting Iraq could be attacked without UN authorisation, when he was warning at the time that such a move would be illegal.
Mr Blair said he was also "uncomfortable" at the time but was trying to make the "political" case for military action, rather than a "legal declaration".
Asked if Lord Goldsmith's legal doubts constrained him from making a commitment to the US, Mr Blair said "No", adding that airing legal doubts at that time would have damaged the coalition and encouraged Saddam.
He said he was convinced that if Lord Goldsmith spoke to Sir Jeremy Greenstock, the UK's then ambassador to the UN, and to "the Americans" he would change his mind on the legality of war, which turned out to be the case.
Mr Blair issued a 26 page written statement ahead of his appearance in response to more than 100 detailed questions from the inquiry panel, in which, among other things, he set out the process by which he said Lord Goldsmith changed his mind.
The inquiry also released a note from Mr Blair to Jonathan Powell, his chief of staff, shortly before his visit to then US President George Bush's ranch in Crawford, Texas, in April 2002, in which he argued that Labour should be "gung-ho" about dealing with Saddam Hussein.
Giving his reaction to Mr Blair's appearance, Former Lib Dem leader Sir Menzies Campbell said Mr Blair's "evangelical, even messianic, determination" to confront Saddam Hussein meant he had ignored anyone with misgivings.
The public were not given the "full information" about the extent of division in the government over the issue, he told the BBC.
SNP leader and Scottish first minister Alex Salmond accused Mr Blair of weaving a "tapestry of deceit".