The Ethiopian military and its allies are responsible for widespread sexual violence against women in Tigray, using rape as a war strategy.
The human rights group says that the scale of violations during the nine-month conflict in the north of the country amounts to war crimes.
One woman reported being gang-raped in front of her children. Ethiopian officials have not responded to the allegations.
Amnesty says “overwhelming evidence” shows sexual violence has been rampant since the very first days of the conflict.
It began last November when the region’s Tigray People’s Liberation Front (TPLF) party stormed a military base after falling out with the prime minister over his political reforms.
The rights group interviewed 63 women and children from Tigray who said they had been raped by Ethiopian or Eritrean soldiers or pro-government fighters belonging to forces and militias from the neighbouring region of Amhara.
“The level of sadistic and gratuitous brutality in addition to the rape was absolutely shocking,” Donatella Rovera, author of the report, said.
A 39-year-old woman reported being seized by Eritrean soldiers as she was travelling with her two children. “Five of them raped me in front of my children,” she told Amnesty.
“They used an iron rod, which is used to clean the gun, to burn me. They inserted pieces of metal in my womb; that was what hurt me. Then they left me on the street.”
Some women Amnesty interviewed said they had been detained for weeks and repeatedly raped, often by several men.
More than half of the women accused Eritrean soldiers of carrying out the violations, identified by their Tigrinya accents and uniforms.
Amnesty is calling on the UN to urgently send a team of experts to Tigray to investigate further the allegations that may amount to crimes against humanity.
Ethiopian officials contacted by the BBC did not respond to requests for comment. Amnesty’s calls to Ethiopian and Eritrean authorities were also ignored.
The widespread nature of the assaults suggests military officials knew what was happening and that it was being tolerated at the highest level of government in Ethiopia and Eritrea, Amnesty said.
“The patterns of sexual violence emerging from survivors’ accounts indicate that the violations have been part of a strategy to terrorise, degrade, and humiliate both the victims and their ethnic group,” the report said.
Ms Rovera said the victims of the assaults have not had access to the support they need to recover, and many live in poverty in Sudanese refugee camps or camps in Tigray.
On Tuesday, when Ethiopia’s prime minister called on civilians to join the army to fight in Tigray, he also accused Tigrayan forces, made of up the TPLF and its allies, of recruiting child soldiers, raping women and blocking aid.
Amnesty says it has not interviewed any women who accuse Tigrayan forces of rape but said it would monitor the situation now the conflict had spread beyond Tigray’s borders.
The TPLF, which has been designated a terrorist organisation by the government but says it is the legitimate regional government of Tigray, has accused the government of using “venomous rhetoric” against Tigrayans and of being responsible for blocking aid.