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.
Chadema officials 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.
“We would like the police to come out and tell us where our chairman is and the reasons for his arrest,” their statement says.
The police have not issued any statement on his whereabouts.
Tanzania’s main opposition party also condemned human rights violations in the East African nation, saying: “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.