The leader of Tanzania's main opposition party Chadema and other members were arrested early Wednesday ahead of a planned conference to demand constitutional reforms, the party said.
Freeman Mbowe and 10 Chadema members were rounded up in the dead of night in the northwestern port city of Mwanza, it said on Twitter.
"We condemn the repression of the rights of Tanzanians with the strongest force. These are signs that the dictatorship that existed during the rule of President John Magufuli continues," the party charged.
"Freeman Mbowe was accosted by an army of police officers in his hotel when he arrived at 02:30 am and was arrested together with other leaders," it said.
While the other Chadema members were taken to Mwanza police station, there was no information about Mbowe's whereabouts.
"We want the police to come out and say where the chairman is and why he was arrested," Chadema said.
The arrests come four months after Tanzania's first female President Samia Suluhu Hassan took office in March following the sudden death of her predecessor Magufuli.
There have been high hopes that President Samia would usher in a change from the autocratic rule of her predecessor, who was nicknamed the "Bulldozer" for his uncompromising leadership style.
The arrests took place after Mbowe vowed to go ahead with a meeting on constitutional reforms despite Mwanza provincial authorities banning public gatherings to contain the spread of coronavirus.
"We cannot continue with the old order," Mbowe said in a video published on Twitter on Monday, dressed in a red shirt and beret.
"We have the right to meet but are arrested, beaten, accused and taken to court for two to three years and then freed.
"If they want to arrest all members of the Chadema party, let them first expand the jails because we are all ready to be arrested and will not request bail."
In April, Samia had reached out to the opposition and vowed to defend democracy and basic freedoms in the East African country, which had seen a slide into autocratic rule under her predecessor.
In November 2020, several top opposition leaders including Mbowe were briefly detained after calling for mass protests against what they charged was a rigged election that returned then president Magufuli to power for a second term.
The deeply Covid-sceptic Magufuli died in March of what the authorities said was a heart condition but his political opponents insisted he had coronavirus.
Tanzania was long seen as a haven of stability and democracy in an otherwise volatile neighbourhood, but alarm grew over Magufuli's increasingly authoritarian rule.