NAIROBI, April 12, 2025 (BSS/AFP) - Tanzania must immediately and unconditionally release opposition leader Tundu Lissu, Amnesty International said Friday, after he was arrested and charged with treason.

The east African nation has increasingly cracked down on its opposition, with Lissu's Chadema party accusing President Samia Suluhu Hassan of returning to the repressive tactics of her predecessor months before October's general election.

Lissu and other party members were detained on Wednesday after attending a rally in Mbinga, a town in the southern Ruvuma region, where Chadema said police officers dispersed the crowds with tear gas.

The opposition leader was later taken to a court in economic capital Dar es Salaam on Thursday and charged with treason, a capital offence.

"The Tanzanian authorities must immediately and unconditionally release Tundu Lissu," Tigere Chagutah, Amnesty International's regional director for East and Southern Africa, said in a statement late Friday.

A "campaign of repression" by authorities against the opposition had seen "four government critics forcibly disappeared, and one unlawfully killed in 2024", he said.

Chagutah said police had subjected opposition members to "mass arrest, arbitrary detention and unlawful use of force", criticising the "heavy-handed tactics to silence critics".

He urged Tanzania to "focus on upholding fundamental human rights in the country, including the right to freedom of expression and peaceful assembly".

Hassan was initially feted for easing restrictions that her predecessor John Magufuli had imposed on the opposition and the media in the country of 67 million people.

But rights groups and Western governments have criticised what they see as renewed repression, with the arrests of Chadema politicians as well as abductions and murders of opposition figures.