In his long-awaited appearance before the Iraq Inquiry, the former prime minister denied he had taken the country to war on the basis of a "lie" over Saddam Hussein's supposed weapons of mass destruction (WMD).
He suggested the world could now be faced with the threat of a nuclear-armed Iraq if he and President George Bush had not taken action to confront the Iraqi dictator.
Asked at the end of six hours of testimony by inquiry chairman, Sir John Chilcot, whether he had any regrets, he said: "Responsibility but not a regret for removing Saddam Hussein.
"I think that he was a monster. I believe he threatened not just the region but the world. And in the circumstances that we faced then, but I think even if you look back now, it was better to deal with this threat, to remove him from office."
One member of the audience shouted out: "What, no regrets? Come on".
Then as he left, another audience member heckled: "You are a liar," while another added, "And a murderer".
His voice apparently beginning to fade, after what had been a largely assured and fluent performance, he insisted that Britain - and in particular the armed forces - should feel an "immense sense of pride" for the role they had played.
"I had to take this decision as prime minister. It was a huge responsibility and there is not a single day that passes by that I don't reflect and think about that responsibility and so I should," he said.
"But I genuinely believe that if we had left Saddam in power, even with what we know now, we would still have had to have dealt with him, possibly in circumstances where the threat was worse.
"In the end it was divisive and I am sorry about that and I did my level best to bring people back together again but if I am asked whether I believe we are safer more secure, that Iraq is better, that our own security is better, with Saddam and his two sons out of office and out of power, I believe indeed we are."
At the end of a gruelling day some members had to be led out of the hearing room sobbing, after breaking down in tears.
Despite an uncharacteristically nervous sounding start, Mr Blair rarely appeared discomforted during more than six hours of questioning.
However his appearance infuriated some of the families of troops killed in the conflict sitting in the audience, who accused him of being "smug" and "smarmy".
Mr Blair admitted that it had always been his intention that Britain would be "shoulder to shoulder" with the Americans if it came to war.
However he rejected suggestions that he had struck a secret deal with Mr Bush to overthrow Saddam insisting that he had always been open about what he was doing.
"The one thing I was not doing was dissembling in that position. The position was not a covert position, it was an open position," he said.
"This isn't about a lie or a conspiracy or a deceit or a deception. It's a decision."
Mr Blair arrived almost two hours early for the hearing to avoid the determined pack of several hundred anti-war protesters gathered outside the Queen Elizabeth II Conference Centre in Westminster.
From the outset he emphasised that his thinking on the threat posed from by Saddam had changed "dramatically" following the 9/11 attacks in 2001.
"I never regarded September 11 as an attack on America, I regarded it as an attack on us. And I had said we would stand shoulder to shoulder with them," he said.
While he agreed that Britain had never accepted any link between Iraq and al Qaida, he was not prepared to take the chance that Saddam would could re-constitute his WMD programmes given his support for other terrorist organisations in the Middle East.
When he met Mr Bush at the president's Texas ranch in Crawford in April 2002, he said they had agreed on the need to confront the Iraqi dictator, but insisted they did not get into "specifics".
"What I was saying - I was not saying this privately incidentally, I was saying it in public - was 'We are going to be with you in confronting and dealing with this threat'.
But pressed on what he thought Mr Bush took from their meeting, he went further, saying: "I think what he took from that was exactly what he should have taken, which was if it came to military action, because there was no way of dealing with this diplomatically, we would be with him."
Mr Blair insisted if Iraq could pose an even more deadly threat - possibly locked in a nuclear arms race with Iran - if Britain and the US had "lost out nerve" and not stopped Saddam.
"Sometimes what is important is not to ask the March 2003 question, but to ask the 2010 question," he said.
"What we now know is that he retained absolutely the intent and the intellectual know-how to restart a nuclear and a chemical weapons programme.
"I have little doubt myself ... that today we would be facing a situation where Iraq was competing with Iran, competing both on nuclear weapons capability and competing more importantly perhaps than anything else, in respect of support of terrorist groups."
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