The human rights group said it "believes that the death toll is significantly higher" and was still investigating. It called on the international community to condemn the bloodshed.
At least 143 demonstrators were killed across Iran since leaders ordered security forces to stamp out protests that followed fuel price increases on November 15, Amnesty International said on Monday.
"According to credible reports ... those killed include at least 143 people," said the London-based rights group. "The deaths have resulted almost entirely from the use of firearms."
It said one person reportedly died after inhaling tear gas, another after being beaten. Amnesty "believes that the death toll is significantly higher" and was still investigating, it said.
'Protesters were shot'
Amnesty, which last week gave a death toll of more than 100, called for the international community to condemn the bloodshed.
"The international community's cautious and muted response to the unlawful killing of protesters is woefully inadequate," said Philip Luther, Amnesty's research and advocacy head for the Middle East.
"They must condemn these killings in the strongest possible terms and describe these events for what they are — the deadly and wholly unwarranted use of force to crush dissent."
Amnesty said "verified videos show security forces deliberately shooting unarmed protesters from a short distance. In some cases, protesters were shot while they were running away."
They also showed security forces shooting from rooftops, it said, adding that the crackdown was carried out by police, the Revolutionary Guard, and the Basij paramilitary force "and others."