Blair told Britain's Iraq Inquiry that his contentious decision to back the 2003 U.S.-led invasion of Iraq was inspired by fears of another, even deadlier, terror attack.
"It wasn't that objectively he (Saddam) had done more, it was that our perception of the risk had shifted," Blair said. "If those people inspired by this religious fanaticism could have killed 30,000, they would have. From that moment Iran, Libya, North Korea, Iraq ... all of this had to be brought to an end.
"The primary consideration for me was to send an absolutely powerful, clear and unremitting message that after Sept. 11 if you were a regime engaged in WMD (weapons of mass destruction), you had to stop."
Clutching a sheath of documents, a tense-looking Blair sat down in a London conference center to answer questions from the Iraq Inquiry, a wide-ranging investigation commissioned by the government to scrutinize the behind-the-scenes machinations from 2001 through Britain's decision to join the costly and unpopular Iraq war.
Blair was questioned about charges that his government was so determined to topple the Iraqi dictator that they exaggerated the content of intelligence reports on Iraq's supposed weapons of mass destruction. He was pressed on when exactly he offered U.S. President George W. Bush support for an invasion.
The former British ambassador to Washington, Christopher Meyer, has said it appeared that an agreement was "signed in blood" by Bush and Blair at the president's ranch in Crawford, Texas, in April 2002.
"The only commitment I gave (at Crawford) was a commitment to deal with Saddam," Blair said. He said he told Bush "we will be with them in dealing with this threat."
Blair said other world leaders did not feel the same way he and Bush did.
"Although the American mindset had changed dramatically (after Sept. 11) - and frankly mine had as well - when I talked to other leaders, particularly in Europe, I didn't get the same impression."
An audience gathered in a central London convention center for the session included family members of soldiers and civilians killed or missing in Iraq. Commuters arriving at the Westminster underground station near the hearing center were met by several people gathering signatures for a petition urging that Blair be tried as a war criminal.
Blair had arrived shortly before 0700GMT (0200EST) Friday, dodging demonstrators by entering the conference center through a cordoned-off rear entrance. About 150 protesters clustered outside shouted slogans including "Jail Tony" and "Blair lied - thousands died," as rows of police officers looked on.
As Blair testified, demonstrators outside the convention hall read aloud the names of civilians and military personnel killed in Iraq.
"The Iraqi people are having to live every day with aggression, division, and atrocities," said protester Saba Jaiwad, an Iraqi who opposed the war. "Blair should not be here giving his excuses for the illegal war, he should be taken to The Hague to face criminal charges because he has committed crimes against the Iraqi people."
Blair acknowledged that the decision to join the war - which led to the largest public protests in a generation in London - had met with opposition in the country, and in his own Cabinet.
"The one thing I found throughout this whole matter from a very early stage is that I was never short of people challenging me on it," Blair told the panel.
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Associated Press Writer Gregory Katz contributed to this report
