"This is actually more like the phenomenon of revolutionary communism," Blair said in an interview with ABC News, commenting on the reach of Islamic extremism.
"It's the religious or cultural equivalent of it, and its roots are deep, its tentacles are long, and its narrative about Islam stretches far further than we think into even parts of mainstream opinion who abhor the extremism, but sort of buy some of the rhetoric that goes with it," he added.
Blair said he didn't understand fully the phenomenon at the time of 9/11, when al-Qaeda operatives hijacked planes and crashed two of them into the World Trade Center in New York, killing about 3,000 people."If these people could have killed 30,000 or 300,000, they would have," Blair warned.
The former British prime minister's remarks came just a week before the ninth anniversary of the attacks, which he said changed his outlook and led to perhaps the most controversial decision of his tenure -- his support for the 2003 U.S.-led invasion of Iraq.
Blair said he felt "an enormous responsibility" for the lives lost in the conflict, which was fiercely opposed by many in Britain.
But he said sanctions aimed at crippling Iraqi dictator Saddam Hussein were "crumbling" and there were real fears that his regime could help terrorists acquire weapons of mass destruction.
"My view was in the circumstances after 9/11, you have to send such a strong signal out on this issue," he said.
The former leader took a similarly hard line on Iran, which many in the international community fear is seeking nuclear weapons under the guise of a civilian nuclear energy program.
"I would tell them they can't have it, and if necessary, they will be confronted with stronger sanctions and diplomacy. But if that fails, I'm not taking any option off the table," he said.
Blair said he did not favor war with Iran but added: "I'm saying I think you cannot exclude it because the primary objective has got to be to prevent them from getting a nuclear weapon."