Image This photograph taken from television shows former British Prime Minister Tony Blair speaking before the Iraq War Inquiry on Friday in London. Credit... UKPB, via Agence France-Presse — Getty Images

A third of the seats in the gallery were assigned to relatives of the 179 British servicemen and women killed in Iraq in the six years of conflict that ended for Britain with the withdrawal of its last units in July last year. Many of the family members told reporters afterward that they were bitterly disappointed not to have heard the former prime minister express some contrition over the conflict, or at least over the deaths of British troops.

The inquiry offered Mr. Blair a prominent platform to map out his version of a history that has brought much vilification in his own land. He defended his close relationship with Mr. Bush, depicted by Mr. Blair’s adversaries — and by some of his former aides in their testimony before the inquiry — as having involved a covert plan by Mr. Blair to circumvent hostile opinion both in Britain and at the United Nations.

“This isn’t about a lie or a conspiracy or a deceit or a deception,” Mr. Blair said. “It’s a decision. And 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 had caused, given 10 years of breaking U.N. resolutions, could we take the risk of this man reconstituting his weapons program or is that a risk it is responsible to take?”

Taut and ill at ease at the outset of his testimony, Mr. Blair quickly caught his rhythm, and rode out a series of impatient thrusts by members of the inquiry panel, effectively dominating the hearing. He offered no apology for joining President Bush in toppling Mr. Hussein, saying repeatedly he thought it was in the best interests of Britain and the world.

Image Police watched protests outside the Iraq Inquiry on Friday in London. Credit... Peter Macdiarmid/Getty Images

“The decision I took — and frankly would take again — was: if there was any possibility that he could develop weapons of mass destruction, we would stop him. It was my view then and that is my view now,” he said.