The leader of Tanzania's main opposition party Chadema and other members were arrested early Wednesday, the party said, ahead of a planned conference to demand constitutional reform.
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.
The other Chadema members were taken to Mwanza police station but 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 Hassan would usher in a change from the autocratic rule of her predecessor, who was nicknamed the "Bulldozer" for his uncompromising leadership style.
The arrests came hours 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.