Rounding off his day-long evidence session, Blair said he accepted "responsibility but not a regret for removing Saddam," insisting the Iraqi leader was a "monster" who had "threatened not just the region but the world."
As he left the London hearing, there were shouts of "liar" and "you're a murderer" from the public gallery, where some of the relatives of the 179 British troops killed in Iraq watched his appearance.
In earlier testimony, Blair denied striking a "covert" deal with then US president George W. Bush over the war, but said he had been convinced Iraq had weapons of mass destruction (WMD) and had to be tackled.
"The decision I took -- and frankly would take again -- was if there was any possibility that he could develop weapons of mass destruction, we should stop him," he said.
Almost seven years after the invasion and six months after British troops left Iraq, Blair's decision to go to war remains highly controversial, and hundreds of protesters demonstrated outside the inquiry venue.
Inside, sitting before the panel in a navy blue suit and red tie, Blair gave a typically assured account of his conduct, defending the intelligence that made the case for war and insisting Iraq was a better place without Saddam.
He was asked whether he had pledged Britain's support for war during an April 2002 meeting with Bush at the president's ranch in Crawford, Texas -- 11 months before the British parliament voted for military action.
Blair denied this, saying he had told Bush "we are going to be with you in confronting and dealing with this threat" of Iraqi WMD.
He added: "How we did that was an open question, and even at that stage I was raising the issue of going to the UN."
Blair admitted he and Bush had discussed the military option but only if the UN route failed, adding this was not a "covert position."
Britain needed to be involved because it was potentially threatened by any proliferation of WMD, and Blair said that if military action was the right thing to do, he wanted to be "in there right alongside" Washington.
When no WMD were found in Iraq, Britain faced questions about the intelligence used to make the case for war, including a key September 2002 dossier which claimed Saddam could launch WMD within 45 minutes.
It has since emerged this claim referred to battlefield weapons, and Blair admitted it should have been "corrected".
But he insisted Britain and many other nations believed Iraq had WMD, and that after the September 11, 2001 attacks, he could not risk the possibility Saddam would pass them on to terror groups.
"The crucial thing after September 11 is that the calculus of risk changed," Blair said.
He was asked about a recent TV interview in which he appeared to say he would have invaded Iraq even if he had known WMD would not be found, leading some commentators to suggest he was intent on removing Saddam regardless.
But he insisted: "I didn't use the words 'regime change' in that interview and I didn't mean in any sense to change the basis (for war)."
The invasion was based on UN Security Council resolution 1441, which in November 2002 gave Saddam a final deadline to disarm, but former British government lawyers have said this did not provide a legal basis for war.
Asked about this, Blair insisted: "I think all countries who took military action believed they had a sound legal basis for doing so."
Hundreds of anti-war protesters outside the conference centre where the inquiry is taking place waved placards saying "Bliar" and chanted "Tony Blair war criminal".
Valerie O'Neill, whose son Kris, 27, was killed by a roadside bomb in Basra in April 2007, was angered by Blair's evidence and blasted his refusal to "admit he's made a mistake."
Britain's newspapers Saturday expressed astonishment at Blair's defiant insistence he had no regrets, attacking what they saw as a crude view that divided the world into simple good and bad.
"He believes the Universe is best understood as an eternal struggle between the forces of good and evil, in contention for dominance," said The Times.
The Guardian accused Blair of living on a "strange planet", where "the invasion of Iraq was not a disaster, but a necessary and even heroic act."