"If there was any possibility that he could develop weapons of mass destruction we should stop him. That was my view then. It's my view now," he said.
Despite the fact weapons of mass destruction were never uncovered by UN inspectors, Blair said he remained convinced that Saddam had the capability.
"He [Saddam] had used them, he definitely had them ... and so in a sense it would have required quite strong evidence the other way to be doubting the fact that he had this programme," Blair said.
"The decision I had to take was, given Saddam's history, given his use of chemical weapons, given the over one million people whose deaths he caused, given 10 years of breaking UN resolutions, could we take the risk of this man reconstituting his weapons programme?
"The primary consideration for me was to send an absolutely powerful, clear and unremitting message that after September 11 if you were a regime engaged in WMD [weapons of mass destruction], you had to stop."
The inquiry is Britain's third major investigation into the conflict. It was set up last year by Gordon Brown, who took over from Blair as the UK's prime minister, to learn lessons from the conflict.
'Not tough enough'
Vincent Moss, the political editor of the Sunday Mirror newspaper, told Al Jazeera the inquiry could have been tougher on Blair, whose decision to send 45,000 troops to Iraq sparked protests in Britain and around the world.
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"A lot of ground wasn't covered, and in my mind it wasn't covered in enough detail, particularly the dodgy dossier in September 2002.
"There wasn't very much interrogation on that, they pretty much accepted what Tony Blair said about the intelligence. We could have had an awful lot stronger questioning on that," he said.
In a foreword to the so-called "dodgy dossier", it was said that possession of chemical and biological weapons by Saddam was "beyond doubt" and that he could deploy them within 45 minutes.
But the inquiry has already heard from senior civil servants who said intelligence in the days before the invasion indicated that Saddam's "weapons of mass destruction" had been dismantled.
Blair on Friday dismissed the controversy over the dossier, saying "it was actually received as dull and cautious at the time".
"It really assumed a vastly greater importance at a later time precisely because of the allegation which was an extraordinarily serious one that we, Downing Street, had deliberately falsified the intelligence which of course we hadn't."
Legality of war
The inquiry pressed Blair on evidence given earlier this week by two former legal advisers to the foreign office, who said there was no lawful basis for war.
Michael Wood, one of the former advisers, told the inquiry said that UN resolution 1441, which gave Saddam Hussein a final chance to give up his weapons of mass destruction, did not provide a case for military action.
"Before 2003 there were problems with security, infrastructure and services, and people died because of the sanctions, but after 2003 there are major disasters"
But Blair said the advice of Peter Goldsmith, the attorney general at the time, made clear that 1441 justified the use of force against Saddam.
"What was so important to me about resolution 1441 was not simply that it declared Saddam in breach but it said also that a failure to comply unconditionally and immediately and fully with the inspectors was itself a material breach," he said.
"If [Goldsmith] in the end had said this cannot be justified lawfully, then we would have been unable to take action."
Goldsmith told the inquiry earlier in the week that he had initially believed a second resolution was needed to justify conflict, but changed his mind after consulting with lawyers in the US.
Blair dismissed any suggestions that Goldsmith had been pressured into changing his opinion, saying: "Anybody who know Peter knows he would not have done it unless he believed in it and thought it was the correct thing to do."
'No success' in Iraq
Ahmed Rushdi, an Iraqi journalist, told Al Jazeera he was not surprised by Blair's defence of the invasion.
"A liar is still a liar," he said.
Rushdi also said that removing Saddam had done little to help ordinary Iraqis.
"Before 2003 there were problems with security, infrastructure and services, and people died because of the sanctions, but after 2003 there are major disasters," he said.
"Major blasts have killed about 2,000 people up till now. After six years or seven years there is no success on the ground, in any aspect."
Blair acknowledged that Britain bore some "responsibility" for the breakdown in security in Iraq since the invasion.
"But let's be quite clear why we faced the difficulty ... because these people were prepared to go and kill any number of completely innocent people," he said.
Hundreds of protesters, including anti-war campaigners and the families of some of the 179 soldiers who have died in Iraq, have gathered outside the building where Blair is giving evidence.
As Friday's inquiry opened John Chilcot, the panel's chairman, reminded the audience that the hearing "is not a trial".