Maria Sarungi Sehai, a prominent Tanzanian activist kidnapped by armed men in Kenya, said she was released hours after the incident.
Amnesty International Kenya spokesman Roland Ebole told the BBC he was “forced” into a car in the capital Nairobi on Sunday afternoon.
But Ms Sehai was released after a few hours. She shared a video to her 1.3 million followers on X, looking visibly shaken and emotional, but saying: “I'm saved.”
Ms Sehai is a staunch critic of Tanzanian President Samia Suluhu Hassan, and has accused his government of “bringing back tyranny” to the country.
President of the Law Society of Kenya, Faith Odhiambo, told X that they had managed to arrange his release.
“We are sending a warning. We will not allow our country to be used as a safe haven to pick up people,” he told a press conference in the evening.
Neither Kenyan nor Tanzanian officials have commented.
Ms Sehai is a fierce advocate for land rights and freedom of expression in Tanzania.
There are concerns that Tanzania could return to the repressive regime of the late President Magufuli, despite his successor lifting the ban on anti-Samiya rallies and promising to restore competitive politics.
Last year, dozens of opponents were arrested and some were brutally killed. A senior opposition leader died in acid attack.
Human Rights Watch described the increase 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 Sehai, said in a statement on X that it believed Tanzanian security agents were “operating beyond Tanzanian 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 raised concerns about her safety, reporting an incident where two unidentified men sought her out at her home while she was away.
Kenya has a history of enabling foreign governments to abduct and forcibly extradite its citizens in violation of international law.
Last year, the Ugandan opposition leader, Kizza Besigye, was kidnapped by Ugandan security officials in Nairobi and taken across the border to be tried by court martial.
The Ugandan government says Kenya helped them in the operation – but the Kenyan government denies this.
Mr Ebole told the BBC “this could be another repeat” of Mr Besigye's situation.
Domestically, Kenya has been gripped by a wave of disappearances, following last year's youth-led protests against planned tax increases.
A state-funded rights group says more than 80 people have been abducted in the past six months.
A few have been released in recent weeks, and there are growing calls for the release of those abducted.