In a report published on Thursday, the London-based group estimated that more than 10,000 civilians had been killed since the invasion began on 20 March last year.
Condemning "flagrant violations" of human rights in occupied Iraq, Amnesty said, "One year after the war on Iraq was launched, the promise of improved human rights for Iraqi citizens remains far from realized."
"The past year has seen scores of unarmed people killed due to excessive or unnecessary use of lethal force by coalition forces during public demonstrations, at checkpoints and in house raids," it said.
"Thousands of people have been detained, often under harsh conditions, and subjected to prolonged and often unacknowledged detention. Many have been tortured or ill-treated and some have died in custody."
It said that "scores of civilians have been killed apparently as a result of excessive use of force by US troops, or have been shot dead in disputed circumstances."
US troops have used excessive
force against Iraqi civilians
Apart from compensation paid to victims' families, "no US soldier has been prosecuted for illegally killing an Iraqi civilian," the organisation noted.
"Iraqi courts, because of an order issued by the US-led authority in Baghdad in June 2003, are forbidden from hearing cases against US soldiers or any other foreign troops or foreign officials in Iraq," said Amnesty.
"In effect, US soldiers are operating with total impunity," it said.
"Violence is endemic, whether in the form of attacks by armed groups or abuses by the occupying forces," it said, accusing resistance fighters of targeting civilians as well as the occupation forces of "crimes against humanity".
"The human rights situation remains critical," concluded Amnesty, whose report was compiled from numerous visits to Iraq since the launch of the invasion.
"For the next year to be better than the last, the occupying forces, Iraqi political and religious leaders and the international community must make a real commitment to protecting and promoting human rights in Iraq," Amnesty said.
"Iraqi civilians are still being killed every day."
"In effect, US soldiers are operating with total impunity"
More than 10,000 Iraqi civilians are estimated to have been killed "as a direct result of the military intervention in Iraq, either during the war or during the subsequent occupation," it said.
"The figure is an estimate as the authorities are unwilling or unable to catalogue killings."
Amnesty said the US-led occupation recognised holding around 8,500 detainees. "However, one Iraqi human rights organisation put the number of detainees at 15,000," it said.
"Many detainees have alleged they were tortured and ill-treated by US and UK troops during interrogation," the report said.