Tanzania has released a Kenyan activist who had been arrested after he tried to attend a hearing in the treason case against a detained opposition leader in Dar es Salaam, Kenyan authorities said on Thursday.

The Kenya National Commission on Human Rights (KNCHR) said it had received Boniface Mwangi in Kenya's southeastern Kwale County on Thursday afternoon. Kenya's foreign minister, Musalia Mudavadi, also confirmed Mwangi's release.

Mwangi had attempted to observe opposition leader Tundu Lissu's first court appearance on Monday, in a case that has spotlighted a growing crackdown on opponents of President Samia Suluhu Hassan.

Lissu, who was shot 16 times in a 2017 attack and came second in Tanzania's last presidential poll, was charged with treason in April over what prosecutors said was a speech calling on the public to rebel and disrupt elections due in October.

Mwangi was arrested on allegations of providing false information to gain entry into the country, an advocacy group said.

"(Mwangi) is in high spirits. The Commission and other partners are making arrangements to transfer him to Nairobi for medical attention," KNCHR wrote on X, sharing a photograph of Mwangi alongside rights activist Hussein Khalid.

Khalid said Mwangi had been severely tortured by Tanzanian security agents and could barely walk: "His body is broken but his spirit remains strong."

The chief spokesperson for Tanzania's Immigration Services Department, Paul Mselle, did not immediately respond to a message requesting comment on the torture accusations.

It was not clear whether Ugandan lawyer and activist Agather Atuhaire, who was being held with Mwangi, had also been released.

President Hassan, who is seeking re-election in October, has said Tanzania's government is committed to respecting human rights following a series of high-profile arrests of political opponents.

But in public remarks on Monday, she warned foreign activists against "invading and interfering in our affairs".