Atuhaire, who won an International Women of Courage Award from the United States last year, was arrested on Monday in Tanzania's business hub Dar es Salaam.

She had travelled there to support opposition leader Tundu Lissu, who is on trial for treason, facing a potential death penalty, ahead of elections in October.

Atuhaire was abandoned early Friday by Tanzanian agents near the Ugandan border after a brutal ordeal, she said.

"What happened in Tanzania stays in Tanzania," she said she was told. "We have videos of you."

Atuhaire was arrested along with Boniface Mwangi, a well-known rights activist from Kenya who also wanted to attend the trial.

Police told her: "Whites are sending you to destabilise our country," she told AFP in an interview in the Ugandan capital Kampala on Friday.

There, they took Mwangi out of the car and began beating him.

"He was screaming," said Atuhaire, adding that the agents had played gospel songs on the car radio, apparently trying to muffle the sound.

She says she was stripped naked, her hands cuffed to her ankles. She has injuries on her forearms and legs.

One of the Tanzanian officers then hit the soles of her feet "with all his might", while another inserted an object into her anus, she said.

"I had never known pain like that existed," she said.

"I don't remember which pain was worse," she added. "After that beating, I scream, I scream."

Then they smeared her body with excrement, she said.

The whole scene was filmed -- "to humiliate, instil fear but also silence you", she said.

"They are used to sexual abuse being something a victim is ashamed of. (But) I am not that victim... I am not the one who should be ashamed. You are the one who is committing a heinous crime, so you are the one who should be ashamed," Atuhaire said.

The US State Department said Saturday it was "deeply concerned" about the reports of mistreatment of Atuhaire and Mwangi, calling for "an immediate and full investigation".

Amnesty International also said the "torture and forcible deportation" of Mwangi and Atuhaire must be "urgently investigated".

AFP attempted to reach the Tanzanian government for comment, but there was no immediate response.

'Pain was unimaginable'

Atuhaire, a lawyer and journalist, is a fierce critic of the government of Ugandan President Yoweri Museveni, who has ruled the country for almost 40 years.

Her work in exposing corruption as head of the Agora Centre for Research has earned her international recognition.

Tanzanian President Samia Suluhu Hassan said Monday foreign activists were attempting to "intrude and interfere" in the country's affairs.

She urged the security services "not to allow ill-mannered individuals from other countries to cross the line here".

Rights groups accuse Hassan of a brutal crackdown on the opposition ahead of the October elections.

After being interrogated, Atuhaire and Mwangi were blindfolded and driven to an unknown location.