The leaders of Tanzania's main opposition party Chadema have been released on bail, a party spokesman said on Tuesday, after they were detained in a mass roundup ahead of a youth day rally.

As many as 520 people were arrested across the country, according to a police statement, before a banned Chadema rally on Monday that had been expected to draw thousands of young people in the southwestern city of Mbeya.

But Chadema posted on X on Tuesday that its offices in Mbeya "are surrounded by the police and they are not allowing people to enter the offices".

Rights groups and government opponents have raised fears the police action could signal a return to the oppressive policies of Tanzania's late president John Magufuli.

The arrests came despite his successor President Samia Suluhu Hassan vowing a return to "competitive politics" and easing some restrictions on the opposition and the media, including lifting a six-year ban on opposition gatherings.

Those released included Chadema chairman Freeman Mbowe and his deputy Tundu Lissu -- both former presidential candidates -- and other top party officials, Chadema spokesman John Mrema said on X.

"(They) have been returned to Dar es Salaam by police and have bailed themselves out", he said but added that "some leaders" continue to be held, without giving further details.

Awadh Haji, police chief of operations and training, said "all the top Chadema leaders who were arrested, after interrogation and other procedures, have been returned to where they came from".

He warned that police would "take strict legal action against any individual or group involved in disrupting peace".

Officers will continue to closely monitor the situation, he said, and will "strengthen security in the city of Mbeya and all other regions of Tanzania to prevent any planned acts of violence".

Mbowe, 62, was detained on Monday at the airport in Mbeya, the day after several other leaders including Lissu were detained.

Hundreds of youth supporters were also rounded up by police as they travelled into the city, according to the party. About 10,000 had been expected to meet in Mbeya to mark International Youth Day on Monday.

But police accused Chadema of planning violent demonstrations and made reference to widespread anti-government protests in neighbouring Kenya, led largely by young activists.

Rights groups and government opponents were alarmed at the police action as Tanzania gears up for local and national elections.

"The mass arrests and arbitrary detention of figures from the Chadema party, as well as their supporters and journalists, is a deeply worrying sign in the run-up to local government elections in December 2024 and the 2025 general election," Amnesty said in a statement.

"The Tanzanian authorities must urgently respect people's rights to freedom of expression and association."

Lissu, 56, a fierce critic of the ruling Chama Cha Mapinduzi (CCM) party, has been arrested multiple times and survived an assassination attempt in 2017.

He returned to Tanzania soon after Hassan lifted the ban on opposition rallies in 2023.

He had spent the previous five years largely in exile, returning only briefly to run for the presidency in 2020.

Mbowe was arrested in July 2021 ahead of a party meeting to demand constitutional reforms and freed the following March after prosecutors dropped terrorism charges against him.