Tanzanian police have arrested prominent politician Tundu Lissu and two other opposition Chadema officials, the party has said.
It says the three were detained on Sunday in the south-western town of Mbeya, on the eve of their planned rally to mark the World Youth Day.
The reported arrests come a year after President Samia Hassan lifted a ban on opposition gatherings and promised to restore competitive politics.
On Sunday the police banned the Chadema rally saying it was intended to cause violence.
They cited the party’s rallying call for people to come together like the “youth in Kenya” – apparently referring to the weeks of deadly anti-government demonstrations in the neighbouring East African country.
On Monday morning, a Chadema official told the BBC that they did not know where the detained leaders were being held.
The police have not officially spoken about the arrests. The BBC has contacted them for comment.
Chadema spokesman John Mrema said groups of youths travelling in hired buses were arrested and forced back to their respective regions under armed escort.
About 10,000 young people were set to meet in Mbeya to celebrate youth day, with the slogan, “Take charge of your future”, according to the party’s youth wing.
But police said in a statement that no such meetings would be allowed in the city or elsewhere in the country.
Chadema officials shared pictures of a large group of about 100 party members who they said had been detained in Iringa, a town close to Mbeya. They were later released and escorted back to where they had come from.
Chadema has condemned the arrests, with party chairman Freeman Mbowe demanding in a post on X “the immediate, unconditional release of our leaders, members, and supporters arrested across the country”.
Mr Lissu, who survived an assassination attempt in 2017 after being shot 16 times, returned to Tanzania last year after two years in exile in Belgium.
He returned after President Samia lifted a ban on political rallies imposed by former President John Magufuli in 2016.
The ban had prohibited political parties from holding rallies and even engaging in internal political activities.
Ms Samia, who came to power in 2021 following Mr Magufuli’s sudden death, was praised for moving away from many of her predecessor’s policies.
But in the wake of the arrests, some opposition politicians have criticised her, saying she is not ready for political reconciliation.
Tanzania is due to hold presidential and parliamentary polls late next year.
