Tanzania's main opposition leader Tundu Lissu told his supporters to have no fear as he appeared in court on Monday for the first time since his arrest on charges that include treason.

Lissu refused to participate in a hearing on April 24 because authorities conducted a virtual, rather than an in-person trial, with him appearing via video link from prison.

On Monday he entered the court with his fist raised in the air as supporters chanted "No Reforms, No Election", according to a video of the courtroom shared by his CHADEMA party on X.

"We will be fine. You should not fear," Lissu said as he took his place in the dock, waving victory signs.

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

A series of high-profile arrests has highlighted the rights record of President Samia Suluhu Hassan, who plans to seek re-election. Hassan says the government is committed to respecting human rights.

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

Several Kenyan rights activists, including a former justice minister, said they were denied entry to Tanzania as they travelled to attend the trial.

Kenya's former Justice Minister Martha Karua, a prominent lawyer and opposition politician, and former Chief Justice Willy Mutunga were among those detained when they landed at Tanzania's Julius Nyerere International Airport in Dar es Salaam, they said on X.

Tanzania's immigration spokesperson Paul Mselle did not immediately respond to requests for comment.

"Today was going to be a big day and we went out there in solidarity," Karua told Kenyan broadcaster NTV on Monday after she was denied entry and sent back to Nairobi.

"The state cannot be used as a personal tool. You cannot deport people whom you don't like, who are not aligned to your views."

Mutunga and rights activist Hussein Khalid were being held in an interrogation room at Julius Nyerere airport on Monday and expected to be deported, Khalid said on X.