Tanzanian riot police detained a number of protesting supporters of arrested opposition leader Freedom Mbowe on Thursday, as a terrorism case against him was postponed.

Mbowe and other officials from the main opposition party Chadema were arrested last month ahead of a planned conference to demand constitutional reform.

The 59-year-old has been charged with terrorism financing and conspiracy in a case that has triggered concern among rights groups and some Western nations about the state of democracy under Tanzania's new leader.

Mbowe had been due to appear in court in the financial capital Dar es Salaam on Thursday via a video link from prison but the case was postponed to Friday because of connection problems, his defence lawyer Peter Kibatala said.

Chadema supporters waving placards saying "Mbowe is not a terrorist" and "Free Freeman Mbowe" gathered outside the court.

Police responded by arresting protesters, the party said on Twitter. Images from the scene showed helmeted police bundling people into a pickup truck and taking them away.

It was not immediately clear how many were detained.

Chadema also said police had raided its regional office in the capital Dodoma on Wednesday night and assaulted a guard before making off with documents.

The party's secretary general John Mnyika urged supporters to turn up at the court again on Friday when Mbowe is due to appear in person. "Going to court is not a criminal offence," he said on Twitter.

Chadema party chairman Freeman Mbowe has been charged with terrorism financing and conspiracy
Chadema party chairman Freeman Mbowe has been charged with terrorism financing and conspiracy AFP / Ericky BONIPHACE

Mbowe's lawyer Kibatala told AFP he was "saddened by the massive use of force" against the protesters.

"All of them were very orderly. They were expressing solidarity and support," he said.

He added the charges against Mbowe "have no basis in law".

"They are opportunistic and probably politically motivated."

Mbowe's arrest came four months after Tanzania's first female president, Samia Suluhu Hassan, took office following the sudden death of her predecessor, John Magufuli.

There had been hopes that Hassan would usher in change from the autocratic rule of her predecessor, nicknamed the "Bulldozer" for his uncompromising style.

Tundu Lissu, who was Chadema's candidate in the presidential election last October but took refuge in Belgium after losing to Magufuli, said Thursday's arrests reflected a deepening slide into "dictatorship."

"Only thoroughgoing constitutional reforms will save Tanzania from impending political & social turmoil," Lissu said on Twitter.

Prosecutors say the terrorism charges against Mbowe do not relate to the constitutional reform forum Chadema had planned to hold in the northwestern city of Mwanza last month, but to alleged offences last year in another part of Tanzania.

Amnesty International has joined the calls for his release, saying the government must substantiate the charges against him.

"Since President Samia Suluhu Hassan's inauguration, the Tanzanian government has taken some encouraging steps towards allowing greater freedom of expression and association in the country," Amnesty said in a statement on Wednesday.

"This case is a concerning development that casts doubt on whether that progress will continue or whether repression will once again be the order of the day."