They labelled the report by former top civil servant Lord Richard Butler a "whitewash" and say it has left the British leader with little credibility.
Released on Wednesday, the report found major holes in pre-war intelligence but spared the British prime minister any personal responsibility.
It was the fourth investigation in a year into the government's justification for war from which Blair has escaped with little more than a slapped wrist.
The report concluded Iraq almost certainly did not possess significant stocks of weapons of mass destruction before the conflict, despite government claims.
But it said Blair was not responsible for the failures of British intelligence and did not intentionally lie to the British people.
Lindsey German, spokeswoman for the Stop the War Coalition, told Aljazeera.net the government had found itself "not guilty".
"All these reports seem to conclude that no one is responsible or to blame for the illegal and unjustified invasion of Iraq which has killed thousands of people," she said.
"But no one should be surprised. The Butler committee was set up to get the right result. This is the government investigating itself and then acquitting itself."
Butler's report says Blair did not
intentionally mislead the public
German said the report committee, which was appointed by the government, had too narrow a remit.
"We in the anti-war movement have been saying for two years that the UK went to war on a false pretext and this report completely vindicates us.
"But no one has ever gone to war solely on the basis of intelligence because it is known that intelligence is never 100% certain. This is the issue that needs to be addressed," German said.
"The Butler committee was set up to get the right result. This is
the government investigating itself and then acquitting itself"
Stop the War Coalition
The Stop the War Coalition spokeswoman added: "Tony Blair should resign immediately and make a full apology to the British and Iraqi people. Troops should also be pulled out of Iraq. That is the only way he can start atoning for this mess."
For his part, Ahmed al-Sheikh, president of the Muslim Association of Britain, told Aljazeera.net that Tony Blair's political judgment should have been the subject of the inquiry.
"Even if the prime minister did not intentionally lie, his political judgment was lousy," he said.
"Thousands of innocent people have been killed in Iraq and scores of British troops have died there because of his misjudgment."
Following the report's publication, Blair told parliament he believed invading Iraq was the right thing to do.
"No one lied. No one made up the intelligence. No one inserted things into the dossier against the advice of the intelligence services," he said.
"Everyone genuinely tried to do their best in good faith for the country in circumstances of acute difficulty. That issue of good faith should now be at an end."
Polls had found the British public
overwhelmingly opposed to war
Blair said while it seems clear Saddam did not have chemical or biological weapons ready to deploy, that did not mean the threat was misconceived and the war unjustified.
"Iraq, the region, the wider world is a better and safer place without Saddam," he said.
Trust in question
Political analysts say while the Labour leader's political career has survived, the war has hammered his public trust ratings.
"Everyone genuinely tried to do their best in good faith for the country in circumstances of acute difficulty. That issue of good faith should now be at an end"
British Prime Minister Tony Blair
"It looks very much like the man they call Teflon Tony has come out of this unscathed again," said John Benyon, politics professor at the University of Leicester.
"But the trust issue is going to continue to be a problem."
And Colin Hay, politics professor at the University of Birmingham, said the debate about the justification for war would not go away.
"The problem for the government is that people are still talking about this. What the government wants to do is to close this issue down and focus on other things.
"But in terms of the overall popularity of the government, I
think the damage has already been done. In a way Butler merely confirms what most people actually now think ... that the government has betrayed public confidence."