Tanzania's President Samia Suluhu Hassan has warned activists from neighbouring Kenya that she will not allow them to "meddle" in her country's affairs and cause "chaos".

Her comments came after prominent Kenyan lawyers and rights campaigners were deported, preventing them from attending the court case of an opposition leader charged with treason.

"If they have been contained in their country, let them not come here to meddle. Let's not give them a chance. They have already created chaos in their own country," Samia said.

Human rights groups are concerned that the Tanzanian government is increasingly cracking down on the opposition ahead of elections in October.

On Monday, the leader of the main opposition Chadema party, Tundu Lissu, appeared in a magistrate's court after being arrested last month.

He raised his fist in a gesture of defiance and told his supporters: "You will be fine. You should not fear."

There was a heavy deployment of security officers at the court in Dar es Salaam, the largest city.

This did not deter Lissu's supporters, hundreds of whom had gathered outside.

The case was adjourned to 2 June after the prosecution asked for more time for investigations.

The court had previously attempted to conduct proceedings virtually, but Lissu's legal team opposed this, citing the need for transparency.

The Chadema party leader was arrested on 9 April following his rallying call for "no reforms, no election".

He is demanding sweeping changes, saying Tanzania's current laws do not allow for free and fair elections. The government denies the allegation..

He has since been charged with treason and publishing false information online, which allegedly incited rebellion and accused police of electoral misconduct.

He denies the charges. If convicted, he could face the death penalty.

Lissu's legal team raised concerns over the heavy police presence around the court, which the magistrate acknowledged, saying there would be consultations on whether this was justified.

President Samia was widely praised for giving Tanzanians greater political freedom when she took office in 2021 following the death of the incumbent, John Magufuli.

But her critics say Tanzania is once again seeing the repression that characterised Magufuli's rule. The government denies the allegation.

The Tanzania Human Rights Defenders Coalition (THRDC) said that Kenyan activist Boniface Mwangi and Ugandan lawyer Agather Atuhaire had been detained after they arrived for Lissu's case.

The chief spokesperson for Tanzania's immigration authority, Paul Mselle, said he was not aware of the arrests, but would look into it, the Reuters news agency reports.

On Sunday, Kenya's former Justice Minister Martha Karua was deported from Tanzania along with two colleagues to prevent them from attending Lissu's court case.

Hours later, former Kenyan chief justice Willy Mutunga and other prominent rights activists said they had been stopped and held at the airport.

But another former Kenyan chief justice, David Maraga, was in court and said that he hoped "the legal process ahead will be fair, just, and expeditious".

Speaking at the launch of Tanzania's new foreign policy in Dar es Salaam on Monday, Samia stressed that she would not tolerate external efforts to undermine her country's peace and stability.

"We will not give a chance to anyone to come and destroy us," she said.

She instructed the authorities not to allow "those who have spoiled their countries to cross over to Tanzania... and spread their indiscipline here".

"I have seen several clips saying I am biased, but what I am doing is protecting my country which is the key mandate I was given," the president added.

Recent events in Tanzania, including the arrest of other opposition figures and a wave of abductions and killings targeting government critics, have sparked condemnation and calls for an end to political repression.

The Chadema party has been disqualified from contesting the general election after refusing to comply with the electoral commission's requirement to sign a code of conduct.