Iraqi insurgents are committing war crimes that undermine any claim they may have to be fighting a legitimate cause, Amnesty International says.
The report coincides with Iraq's bloodiest sequence of attacks
A report by the UK-based group accuses anti-US forces in Iraq of showing "utter disdain for civilian life".
It says there can be no justification for deliberate killings of civilians, hostage-taking and torture.
The perpetrators "place themselves totally beyond the pale of acceptable behaviour", Amnesty International said.
"There is no honour nor heroism in blowing up people going to pray or murdering a terrified hostage," the 56-page report entitled Iraq, In Cold Blood: Abuses by Armed Groups says.
"Those carrying out such acts are criminals, nothing less, whose actions undermine any claim they may have to be pursuing a legitimate cause."
Amnesty acknowledges that many Iraqi citizens agree with the aims of the insurgency, to rid their country of the US-led military presence.
It also stresses that the US and allies have themselves committed grave violations, including killings of civilians and torture of prisoners.
"But abuses committed by one side do not and cannot justify abuses by another," the rights group says.
"This is all the more the case when the principal victims are ordinary Iraqi men, women and children attempting peacefully to go about their everyday lives."
Researchers quote an Iraqi government minister's claim that more than 6,000 civilians have died in insurgency attacks, although the report says the true toll is impossible to calculate, let alone the long-term consequences.
"We urge armed groups to immediately cease all attacks against civilians and all other abuses," Amnesty says.
"Armed groups, like other parties to the conflict in Iraq, are required to comply strictly with international law in all their acts and remain accountable for their actions."
Amnesty International asks religious leaders and other influential figures to make a stand against war crimes and crimes against humanity.
"We earnestly hope that these leaders, by speaking out publicly or through other more discreet means, can help to make the difference," Amnesty says.