Amnesty says total number of Tigrayans arrested by Ethiopian authorities is likely to be in the hundreds, with the whereabouts of many unknown.
Amnesty International has 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 on Friday.
The total number is likely to be in the hundreds, with the whereabouts of many unknown, Amnesty said.
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 that 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.
Ethiopia says aid groups 'arming' fighters
Earlier, Ethiopia's government accused humanitarian aid groups of "arming" Tigray fighters and threatened to halt some groups' operations there.
The accusations reflect the latest frictions between Ethiopia's government and aid groups that for months have sought unrestricted access to the largely cut-off Tigray region where hundreds of thousands of people face famine conditions and scores have starved to death.
The spokesman for the Tigray emergency task force, Redwan Hussein, alleged on Wednesday that aid groups are "playing a destructive role" in the eight-month conflict.
"Instead of coordinating aid, (they) are widely engaged in coordinating, from a distance, campaigns of propaganda to harass and defame the Ethiopian government," he said.
Redwan also said Ethiopia’s government may "reconsider its agreement to work with some of them" if humanitarian workers do not "confine their activities to aid and humanitarian issues."
He didn't name groups.
But the World Food Programme on Thursday responded to a broadcast by the pro-government ESAT news station that alleged the United Nations agency was supportive of the Tigray fighters that have been battling forces from Ethiopia and neighboring Eritrea.
"WFP in Ethiopia and elsewhere in the world strictly adheres to the humanitarian principles of humanity, impartiality, neutrality and operational independence," the agency said, adding that "WFP’s number one priority is to deliver emergency food assistance to vulnerable and hungry communities."
The Tigray forces regained control of much of the region last month including the regional capital, Mekelle, while Ethiopian forces retreated and the government declared a unilateral ceasefire.
Though Ethiopia’s government has said the ceasefire is on humanitarian grounds, aid groups have said access remains severely restricted and both the United States and European Union have compared the situation to a “siege.”
Phone links, internet and electricity have been cut off across most of the region.
Convoys of aid trucks were unable to enter the region for 10 days.
WFP said 50 trucks carrying 900 metric of food and other aid reached the Tigray capital on Monday but warned that “we need 100 trucks to be moving on any given day for half the time if we are to reverse the catastrophic humanitarian situation in the region.”
Ethiopia’s government has denied allegations it is blocking aid to Tigray and says humanitarian flights have been granted permission to fly to Mekelle and the town of Shire, subject to cargo checks.
Meanwhile, PM Ahmed has claimed there is "no hunger" in Tigray.