Tony Blair has said it is time to stand up to the "evil ideology" behind the London bombings and other attacks.
Tony Blair attacked the "devilish logic" of extremist propaganda
Such violence was not in response to any particular policy or founded in any injustice, but in a "fanaticism" that had to be confronted, he said.
Some Labour MPs, including ex-minister Clare Short, said they had "no doubt" the attacks were linked to Iraq.
Meanwhile, police have raided another house in Beeston, Leeds, near the home of London bomber Shehzad Tanweer.
The bomb attacks killed 51 people and injured hundreds more. The four bombers are also thought to have died.
Speaking to Labour party members in London, the prime minister said it would be a "misunderstanding of a catastrophic order" to think that if the developed world changed its behaviour, extremists would change theirs.
"If it is the plight of the Palestinians that drives them, why, every time it looks as if Israel and Palestine are making progress, does the same ideology perpetrate an outrage that turns hope back into despair?
"If it is Afghanistan that motivates them, why blow up innocent Afghans on their way to their first-ever election?
"If it is Iraq that motivates them, why is the same ideology killing Iraqis by terror in defiance of an elected Iraqi government?
"What was 11 September 2001 the reprisal for?"
But some Labour left-wingers said there was a link between the Iraq war and the attacks.
John McDonnell, Labour MP for Hayes and Harlington, said it was "intellectually unsustainable" to say the war in Iraq had not motivated the bombers.
Mohammad Sidique Khan: Aged 30, from Beeston, Leeds, recently moved to Dewsbury, married with baby. ID found at Edgware Road blast site.
Hasib Mir Hussain: Aged 18, lived Holbeck, Leeds. Reported missing on day of bombings. Said to have turned very religious two years ago. ID found in No 30 bus.
Shehzad Tanweer (above): Aged 22, born Bradford, lived Beeston, Leeds. Studied religion in Pakistan. Forensic evidence linking him to Aldgate blast.
Germaine Lindsay: Jamaican-born man who lived in Buckinghamshire.
"For as long as Britain remains in occupation of Iraq the terrorist recruiters will have the argument they seek to attract more susceptible young recruits to the bomb team. Britain must withdraw now."
And in an interview for GMTV's Sunday programme, Labour ex-minister Clare Short, who resigned over the Iraq war, said she "had no doubt" the atrocities were linked to Iraq.
"We are implicit in the slaughter of large numbers of civilians in Iraq and supporting a Middle East policy that for the Palestinians creates this sense of double standards - that feeds anger," she said.
Earlier on Saturday, the family of one of the bombers said he may have been "brainwashed" into carrying it out.
In a statement, they described 30-year-old teacher Mohammad Sidique Khan as "a kind and caring member of our family" and appealed to the public for information.
The family of another of the bombers, 18-year-old Hasib Hussain, have also expressed their "devastation" at events.
British police have turned their attention to finding those who may have helped the bombers carry out last Thursday's attacks.
Officers are searching a house in Leeds linked to Egyptian biochemist Magdi Mahmoud al-Nashar, who has been arrested in Cairo.
Egypt's interior minister said press reports linking Mr al-Nashar to al-Qaeda were "groundless" and based on a hasty conclusion.
Three of the bombers were from West Yorkshire - Hussain, from Holbeck in Leeds; Khan, from Dewsbury, and Tanweer, 22, of Beeston, Leeds, and police are searching their homes.
They are also searching the home of the man they believe is the fourth bomber - Lindsey Germaine, a Jamaican-born man who lived in Aylesbury, Buckinghamshire.
The wreckage of the number 30 bus, which had its roof ripped off in the blast that killed 13 in Tavistock Square, was taken away for further forensic examination on Saturday.
On Friday Britain's top Muslims issued a joint statement of condemnation branding the London bombings "utterly criminal, totally reprehensible, and absolutely un-Islamic".
But Britain's highest ranking Asian police officer, Tarique Ghaffur, said Muslims and their leaders must do more than just condemn the bombings.
Mr Ghaffur, the Metropolitan Police Assistant Commissioner, urged members of the community to inform on potential terrorists and their supporters.
The police would have to engage better with minorities - but the minorities would have to take the first step, he said.