Amnesty International on Friday accused Ethiopia of arbitrarily arresting dozens of ethnic Tigrayans in Addis Ababa and elsewhere since rebels retook control of the war-hit Tigray region's capital last month.
Those detained have included activists and journalists, and some have been beaten and transported hundreds of kilometres (miles) from the capital, Amnesty said.
The total number is likely to be in the hundreds, with the whereabouts of many unknown, Amnesty said.
The report comes as the war in Tigray takes on a new dimension, with the deployment of forces from several regions in the fight against the Tigrayan rebels signalling a potential widening of the conflict.
The fighting has already left thousands dead and, according to the United Nations, pushed 400,000 into famine.
Prime Minister Abiy Ahmed sent troops into Tigray last November to detain and disarm leaders of the region's ruling party, the Tigray People's Liberation Front (TPLF).
He said the move came in response to TPLF attacks on federal army camps.
The 2019 Nobel Peace laureate declared victory in late November after federal troops took the regional capital Mekele, but fighting continued and TPLF leaders remained on the run.
In late June the conflict took a stunning turn when pro-TPLF fighters reclaimed Mekele and Abiy declared a ceasefire.
The recent arrests of Tigrayans outside Tigray began after that, Amnesty said.
"Former detainees told us that police stations are filled with people speaking Tigrinya, and that authorities had conducted sweeping mass arrests of Tigrayans," said Deprose Muchena, Amnesty's East and Southern Africa director.
The arrests should stop and all detainees should be "promptly charged with internationally recognised crimes and given fair trials, or immediately and unconditionally released", Muchena said.
Abiy's government has previously denied engaging in ethnically-motivated arrests.
The federal police and attorney general's office did not immediately respond to a request for comment.
A lawyer for one detainee, who has since been released on bail, told Amnesty his client had been accused of having links to the TPLF, which the government considers a terrorist group.
One detainee told Amnesty that police raided his snooker hall on the night of July 2 and "began to harass and beat customers" before scrutinising identity documents and detaining five Tigrayans.
"They kept us on the open air and it was raining the whole night. We also stayed there the next day on Saturday... We were 26 Tigrayans arrested in the station that day," the man said.
Of those, seven were transported 240 kilometres (150 miles) east, to the Awash Arba area of Ethiopia's Afar region, he said.
On Thursday the Ethiopian Human Rights Commission (EHRC), a state-affiliated but independent body, said it was also monitoring reports of arbitrary arrests, business closures and "other types of harassment targeting ethnic Tigrayans".
The EHRC and rights groups have similarly voiced concern about previous rounds of such arrests going back to the beginning of the war.
Rebels Vow 'Wider' Operations
Tigrayan rebels announced an offensive this week to reclaim disputed territory in western and southern Tigray that was occupied early in the war by security forces from the Amhara region, which borders Tigray to the south.
Thousands of Amhara forces, including militia fighters, have deployed to the border between the two regions, and three other regions have confirmed they are also sending troops.
Spokesmen for both sides have offered competing claims concerning who holds key towns including Alamata in southern Tigray and Mai-Tsebri in western Tigray.
Rebel spokesman Getachew Reda said Friday on Twitter that pro-government forces had been "routed", adding: "We will take all the necessary measures to make sure (Abiy) has no arsenals left to threaten our people."
A statement from the rebels' "military command" said government soldiers and Amhara militia fighters had launched a counter-offensive Thursday near Mai-Tsebri.
It said the rebels' operations would meanwhile "continue in succession and in wider scope than the previous ones".
Separately, Debretsion Gebremichael, president of Tigray's pre-war government, said in a statement Thursday that pro-TPLF forces were not waging war on the population of Amhara.
He insisted they were focused on restoring Tigray's pre-war boundaries.
"You should refuse being part of (this attack)," he said, referring to the government's counter-offensive.
"You should say 'it is enough' and engage in a struggle that would be beneficial to you and your history."
Amhara officials have also stressed that they are looking to reclaim land they accuse the TPLF of illegally annexing in the early 1990s and don't mean to harm Tigrayan civilians.