Tanzanian authorities arrested and later deported human rights activists from Kenya and Uganda who had travelled to Dar es Salaam to observe a hearing in the treason case against detained opposition leader Tundu Lissu, advocacy groups said.

Kenyan activist Boniface Mwangi and Ugandan lawyer Agather Atuhaire went to Dar es Salaam to attend Lissu's first court appearance on Monday in a case that has highlighted what government critics say is a growing crackdown on opponents of President Samia Suluhu Hassan.

The head of Tanzania's Law Society said on Tuesday that Mwangi and Atuhaire had been deported.

"Both individuals have been repatriated under the escort and supervision of officers from the Tanzania Immigration Services Department," Tanzania Law Society President Boniface Mwabukusi said in a statement on his X account.

The chief spokesperson for Tanzania's Immigration Services Department, Paul Mselle, said he was not aware of Mwangi and Atuhaire's arrests, but would look into it. He did not respond when sought for comment about their deportations.

Spokespeople for the government and police did not immediately respond to Reuters' requests for comment.

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.

At Monday's hearing Lissu urged his supporters to have no fear. His lawyer later told reporters that the hearing had been adjourned until June 2.

ARRESTS

The Tanzania Human Rights Defenders Coalition (THRDC) said in a statement late on Monday that Mwangi and Atuhaire were held at the central police station in Dar es Salaam.

It said Mwangi had been arrested on allegations of providing false information to gain entry into the country. It was not clear on what basis Atuhaire had been detained.

Mwangi, who helped lead anti-government protests last year in Kenya, posted on X on Monday that men claiming to be police officers had come to his hotel room and that he would go with them once his lawyers arrived.

Several other Kenyan human rights activists who had hoped to attend Monday's hearing, including a former justice minister, said in social media posts or interviews that they had been denied entry to Tanzania.

President Hassan, who is seeking re-election in October, has said her 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".

Lissu's CHADEMA party has demanded changes to an electoral process they say favours Tanzania's ruling party before they agree to participate in the October ballot.