Tanzanian opposition leader Freeman Mbowe has been arrested, his Chadema party says.
He was in his hotel room on Tuesday night in Mwanza, in the north-western region where he was attending party meetings, when he was detained.
Officials from Tanzania’s main opposition party say they have not been able to establish where Mr Mbowe is being held.
Ten other leaders arrested alongside him had been taken to the Central Police Station in Mwanza, the party said.
According to local media in the East Africa nation, police confirmed the arrests on suspicion of inciting “unauthorised assembly”.
The opposition party has been holding a series of internal meetings in an effort to revive support and had vowed to hold a big political rally to push for the drafting of a new constitution.
Chadema said: “This is a clear sign that the dictatorship under the [late President John Magufuli] still continues.”
Mr Magufuli, who died in March, was accused of oppressing the opposition and curtailing press freedom.
His successor and former deputy Samia Suluhu Hassan has been regarded as more conciliatory and inclusive