Senior leaders of Tanzania's main opposition Chadema party - including its chairman Freeman Mbowe and his deputy Tundu Lissu - have been released on bail, after they were detained ahead of a youth rally.
About 520 people were arrested in a nationwide crackdown to prevent Chadema from holding a parade through the south-western town of Mbeya on Monday.
Police said some of them remained in custody as they had failed to meet the requirements for bail.
The arrests raised fears that Tanzania was returning to the repressive rule of late President John Magufuli, despite his successor Samia Hassan lifting a ban on opposition gatherings and promising to restore competitive politics.
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.
Chadema's youth wing said it had expected 10,000 people to attend rally, under the slogan "take charge of your future".
On Tuesday, the party posted on X that its offices in Mbeya were "surrounded by the police and they are not allowing people to enter".
Chadema spokesman John Mrema confirmed the release of several party leaders - including Mr Mbowe and Mr Lissu - but said that several others remained in detention.
However, police said that "all the top Chadema leaders who were arrested, after interrogation and other procedures, have been returned to where they came from".
Chidema said Mr Lissu was arrested on Sunday, and Mr Mbowe on Monday when he arrived at Mbeya's airport to bail out the party chairman and two other officials, including the leader of the party's youth wing, John Pambalu.
Mr Lissu, who survived an assassination attempt in 2017 after being shot 16 times, returned to Tanzania in 2023 after two years in exile in Belgium.
President Samia, who came to power following Mr Magufuli's sudden death in 2021, 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, questioning her commitment to political reconciliation.
Tanzania is due to hold presidential and parliamentary polls late next year.