Tanzanian Police on Monday night released leaders of Tanzania's main opposition party, Chadema, along with several members, on bail, after holding them for several hours to thwart a planned assembly in Mbeya.

The party had planned a parade to mark the International Youth Day but were confronted with a ban after what police described as an illegal assembly.

Among those freed were party Chairman Freeman Mbowe, Vice-Chairman Tundu Lissu and Secretary-General John Mnyika.

They had been arrested on arrival in the southern Tanzanian city for the event which had been scheduled for Monday.  

The head of the Tanzania police force's operations and training unit, Commissioner Awadh Haji, told a late-night press conference in Mbeya that all Chadema members apprehended in Mbeya during the past two days had been set free.

Read: Tanzania police arrest Chadema leaders, ban meeting

"This includes the party leaders who travelled here from outside the region, most of them from Dar es Salaam. They have been granted police bail and are now in the process of being transported back under tight police escort," CP Haji said without mentioning their names.

Lissu, Mnyika and Chadema central committee member Joseph Mbilinyi were arrested on Sunday shortly after arrival in Mbeya for the forum, and were joined in detention by Mbowe and the party's youth wing chairman John Pambalu the following day after being apprehended at Mbeya's Songwe airport as they touched down from Dar es Salaam.

The latest confrontation between the party and Tanzania's security organs came ahead of nationwide local government elections slated for late October or early November. It followed the ruling CCM party youth wing’s own event commemorating International Youth Day in Zanzibar on August 10.

However, state authorities opted to crack down on Chadema's similar assembly due to concerns that it might trigger anti-government protests along the lines of the Gen Z youth movement in neighbouring Kenya.

The police and Office of the Registrar of Political Parties both issued formal bans on the Mbeya event, citing videos posted on social media by the party's youth leaders, which were deemed to be incendiary.

In an August 8 letter addressed to the Chadema secretary-general, Assistant Registrar Sisty Nyahoza quoted the leaders urging youth nationwide to attend the gathering "in order to show that they are as serious as their Kenyan counterparts in carving a new destiny for the country and doing away with state control."

Read: Tanzania opposition lacking intra-party democracy

 "Such statements indicate a clear intention to incite people and create chaos under the umbrella of celebrating youth day, and as such are prohibited by Tanzania's laws governing political parties activities," Mr Nyahoza said.

CP Haji echoed the same sentiments in a public statement on Sunday, amid reports that dozens of buses carrying hundreds of youth were stopped at various police checkpoints across the country as they made their way towards Mbeya.

On Monday night, the police boss confirmed that up to 520 Chadema members had been arrested and subjected to lengthy interrogation related to the Youth Day event.

"Most of them were arrested on their way to Mbeya for refusing to heed police orders to turn back, and some who were already in Mbeya including the top party officials. All of them, except for those who failed to meet the bail conditions or had other criminal cases, have been escorted back to their home regions," he said.

CP Haji did not disclose the exact numbers of those released last night or those who would remain in custody. 

But he also issued a strong warning that police would not hesitate to take further action against "anyone, regardless of status or prominence in opposition politics, who tried again to break national laws by copying happenings in other countries."