A Kenyan and a Ugandan activist detained in Tanzania after travelling to observe a court appearance by a jailed opposition leader are still in custody, Tanzania's Law Society said on Wednesday, after previously saying they had been deported.

Kenyan activist Boniface Mwangi and Ugandan lawyer Agather Atuhaire had gone to Dar es Salaam to attend opposition leader Tundu Lissu's first court appearance on Monday on charges of treason and publishing false information.

The case has highlighted what government critics say is a growing crackdown on opponents of President Samia Suluhu Hassan.

On Tuesday, Tanzania Law Society President Boniface Mwabukusi had said Mwangi and Atuhaire had been deported. On Wednesday, he issued a clarification saying the two were still in the custody of the immigration department and the society was making efforts to resolve the situation.

"Contrary to initial information, the two individuals have not yet been deported. We have since established that they are no longer in police custody but remain held by the Immigration Department", Mwabukusi said in a statement on Wednesday on his X account.

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

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

Lissu's CHADEMA party has demanded changes to an electoral process they say favours the ruling party.

Hassan, who plans to seek re-election, says her government is committed to respecting human rights. But on Monday she warned foreign activists against "invading and interfering in our affairs," and asked the security services to block them from entering the country.

Several Kenyan activists and lawyers said they were denied entry to Tanzania as they travelled to attend Lissu's hearing.