A Ugandan activist who was arrested and held for days in Tanzania and later found at the border between the two countries is claiming she was assaulted while in detention.

Agather Atuhaire told the BBC that people dressed in plain clothes “blindfolded” her, after which she was hit, “violently” stripped, and sexually assaulted.

Atuhaire had been held incommunicado in Tanzania alongside fellow Kenyan activist Boniface Mwangi, who was on Thursday found at the border with his home country.

The Tanzanian authorities have not commented.

Regional rights groups have called for an investigation and the US Department of State’s Bureau of Africa Affairs said it was deeply concerned by the reports of the two activists’ mistreatment.

“The pain was too much,” said Atuhaire, showing the BBC a scar from where she said she had been handcuffed.

She added that she was “screaming so hard” that they had to cover her mouth.

Atuhaire told the BBC about her alleged assault in graphic detail.

She said she also heard screams from Mwangi, and that those holding him had threatened to circumcise him.

The pair had gone to Tanzania to show solidarity with opposition leader Tundu Lissu, who appeared in court on Monday after being charged with treason

Mwangi recounted his alleged experience in a post on X: “We had been tortured, and we were told to strip naked and to go bathe. We couldn’t walk and were told to crawl and go wash off the blood.”

Despite being allowed into the country, Mwangi and Atuhaire were not permitted to attend the hearing and were arrested.

On Monday, President Samia Suluhu Hassan had warned that she would not allow activists from neighbouring countries to “meddle” in her country’s affairs and cause “chaos”

Atuhaire was found abandoned at the border on Thursday night after being held in custody since Monday, Agora Centre for Research, the Uganda-based rights group that she leads, posted on X.

Mwangi, who was earlier found abandoned on a roadside in northern Tanzania near the Kenyan border, said he had heard Atuhaire “groaning in pain” when they were held together on Tuesday.

The United States has expressed concern over the “mistreatment” of Mwangi and Agather.

In a statement through its Bureau of African Affairs, the United States appealed to all countries in the region to hold those culpable to account