An Amnesty report focuses on testimony from 16 women in the Amhara town of Nifas Mewcha, which was attacked by Tigray forces in mid-August and returned to government control several days later.
Fighters from Ethiopia's Tigray region have gang-raped and abused women in neighbouring Amhara region.
A report released on Wednesday by Amnesty underscored that the year-long conflict between the central government and Tigrayan forces has been marked by allegations of abuses on all sides.
"The testimonies we heard from survivors describe despicable acts by TPLF fighters that amount to war crimes, and potentially crimes against humanity," Amnesty Secretary General Agnes Callamard said.
Sixteen women in the Amhara town of Nifas Mewcha told Amnesty that fighters associated with the TPLF raped them, the report said.
One 45-year-old woman told Amnesty that four TPLF fighters came to her house, demanding coffee.
"I suspected their intentions, and I sent away my daughters," she said, adding the men hurled ethnic insults at her and told her to call back her children.
"'One of them told the others to stop insulting me. He said, 'she is our mother; we don't have to harm her,'" she told Amnesty. "They forced him to leave the house and three of them stayed back at my home. Then they raped me in turns.'"
Amsal Alamrew, the head of the Nifas Mewcha's women and children affairs office, told Reuters that 74 women said they were raped during the nine-day period covered by the Amnesty report. There were likely more victims too afraid or ashamed to come forward, Amsal said.
Sexual violence used as tool in war
Nifas Mewcha was attacked by Tigray forces in mid-August and returned to government control several days later.
Tigray People's Liberation Front (TPLF) spokesperson Getachew Reda said he had not yet read the Amnesty report but told Reuters, "We take such allegations very seriously and we are ready to conduct an independent investigation."
Government spokesperson Legesse Tulu and Amhara spokesman Gizachew Muleneh did not respond to requests for comment.
The United Nations aid chief has said sexual violence has been used as a weapon of war.
Regional Amhara forces entered Tigray in November to support government soldiers when the war erupted. Tigrayan fighters entered Amhara in July after clawing back control of most of Tigray, Ethiopia's northernmost region.