Blair arrived early for the inquiry and entered by a back door amid heavy security and large numbers of police on standby.
Barbara Serra, Al Jazeera's correspondent in London, said: "A lot of people here are disappointed as they wanted to confront Blair when he came in, but he entered through a back entrance."
Speaking to Al Jazeera, Sabah Jawad, an Iraqi activist, said: "We are hoping at best for Tony Blair to admit that he made a mistake, that he regrets the action.
"The action he took in 2003 with president Bush had a devastating impact on the people of Iraq, and Iraq as a country.
"A million people died as a result of this military adventure and the Iraqi people are still suffering daily.
"All the opinion polls still indicate that the British people are very much bitter about what happened to them because they think they have been deceived by Tony Blair.
"By his lies about the weapons of mass destruction, about the link to terrorism and so on."
As Blair testified, demonstrators outside the Queen Elizabeth II conference centre read aloud the names of civilians and military personnel killed in Iraq.
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"The Iraqi people are having to live every day with aggression, division, and atrocities," Saba Jaiwad, an Iraqi protester who opposed the war, said.
"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."
Andrew Murray, chairman of Stop the War Coalition, also said that Blair should face a war crimes trial over the decision to join the US-led invasion in 2003.
"He is an accomplished actor but I think most people have long since seen through the script," he said.
During his evidence, Blair acknowledged that the decision to join the war, which led to the largest public protests in a generation in the UK, had provoked widespread opposition in the country, and in his own cabinet.
He said: "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's appearance will not only affect his own personal legacy but still has the potential to damage Brown, who was finance minister during the war.
Some Labour leaders fear it will reignite strong feelings on the issue among voters, denting support for a party already trailing the Conservative party in polls in the run-up to an election due by June.