Tanzania's opposition leader Tundu Lissu has been charged with treason a day after he was arrested following a rally he held in the south of the country.

The charge is connected to his nationwide campaign pushing for electoral reform under the slogan "No Reforms, No Election".

The country is due to go to the polls in October when Lissu is expected to challenge President Samia Suluhu Hassan.

When Samia first came to power in 2021, after the death of her predecessor John Magufuli, she was praised for reversing some of his more authoritarian tendencies. But she has since been criticised after some opposition members have been targeted with arrests and abductions.

In a series of public appearances, Lissu has been saying that there is no chance of a free-and-fair election in six months unless there are reforms.

The leader of the Chadema party wants the make-up of the electoral commission to change. He has argued that it should not include people appointed directly by Samia.

The authorities have described the campaign as inciting the public against the holding of the general election.

Lissu has been arrested several times in the past.

In 2017, during Magufuli's presidency, he survived an assassination attempt during which he was shot 16 times.

He then went into exile and returned briefly in 2020 to run against Magufuli in that year's election. He left after the results were announced, complaining about irregularities.

He then returned in 2023 after changes that Samia introduced to allow more freedom for the opposition.

Earlier on Thursday, the police used tear gas to disperse Chadema supporters while blocking the party leaders from holding a press conference regarding Lissu's arrest.

Some party supporters told the BBC that nothing would stop them from demanding electoral reforms ahead of the elections.

"We are surprised that the police are harassing us when our rallies are peaceful," one supporter said.

"We know the ruling party, CCM, is behind all this. We will fight for changes before elections."

Rights groups have condemned the use of force and accused the government of using state institutions to silence critics.

A lawyers' association said the arrest of Lissu and the crackdown on his supporters was an abuse of power and showed a lack of political tolerance.

Lissu's treason case has been adjourned until 24 April.