Prominent Tanzanian activist Maria Sarungi Tsehai, who was kidnapped by gunmen in Kenya, said she was released hours after the incident.
Amnesty International Kenya spokesman Roland Ebole told the BBC she was “forced” into a vehicle in the capital Nairobi on Sunday afternoon.
But Ms. Tsehai was released hours later. She shared a video with her 1.3 million followers on X, looking visibly shaken and emotional but saying: “I'm saved.”
Ms Tsehai is a staunch critic of Tanzanian President Samia Suluhu Hassan and has accused her government of returning “tyranny” to the country.
The president of the Law Society of Kenya, Faith Odhiambo, told X that they had managed to arrange her release.
“We are sending a warning. We will not allow our country to be used as a haven to take people away,” she said at an evening press conference.
Neither Kenyan nor Tanzanian officials have commented.
Ms. Tsehai is a fierce advocate for land rights and freedom of expression in Tanzania.
There are fears that Tanzania could return to the repressive rule of the late President Magufuli, although his successor, Samia, lifted a ban on opposition gatherings and promised to restore competitive politics.
Last year, dozens of opposition activists were arrested and some were brutally killed. A senior opposition leader has died after being doused with acid.
Human Rights Watch described the rise in arrests of opposition activists as a “bad sign” ahead of the 2025 presidential election, which will be held in October.
Change Tanzania, a movement founded by Ms Tsehai, said in a statement to X that they believed she had been taken by Tanzanian security agents “operating across Tanzania's borders to silence legitimate criticism of the government”.
It added that her “courage to stand up for justice made her a target”.
In recent months, she has expressed concern for her safety, reporting an incident in which two unidentified men were seen searching her home while she was away.
Kenya has a history of allowing foreign governments to abduct its citizens and carry out forced extraditions in violation of international law.
Last year, Ugandan opposition leader Kizza Besigye was abducted in Nairobi by Ugandan security officials and taken across the border to face a military court.
The Ugandan government said Kenya helped them in the operation, but the Kenyan government denied this.
Mr Ebole told the BBC it “could be another repeat” of Mr Besigye's situation.
Internally, Kenya has been gripped by a wave of disappearances since last year's youth protests against a series of planned tax increases.
A state-funded rights group says more than 80 people have been abducted in the past six months.
Several have been released in recent weeks and there are growing calls for the release of all those abducted.