Lawyers for the leader of Tanzania's main opposition party on Friday accused the government of judicial "interference" and called for terrorism charges against him to be dismissed.
Supporters of Freeman Mbowe and his Chadema party have denounced his arrest as a politically-motivated move to silence dissent, with the case also arousing concern among rights groups and Western nations about the state of democracy under Tanzania's new President Samia Suluhu Hassan.
On Friday, Mbowe's defence lawyer Peter Kibatala filed a petition asking the Kisutu court in Dar es Salaam to dismiss the charges and declare a mistrial, alleging the presidency had meddled in the matter following remarks by Hassan.
The case, which has been postponed to August 27, is currently in the process of being transferred to a higher court.
"We have submitted a notice requesting the Kisutu court to present to the higher court utterances by a government institution which we believe directly interfered (with) the court's independence," Kibatala told journalists outside the courtroom.
"I do not need to mention the government institution because almost everyone heard it," he said.
Mbowe was arrested along with other members of Chadema on July 21, just hours before they were to hold a public forum on constitutional reform.
But in an interview published this week by the BBC, Hassan said the charges against Mbowe were "not political" and were the result of a nearly year-long investigation.
"I suspect that, knowing the charges he was facing, he calculated that if he was arrested he could claim that it was because he was pushing for a new constitution," she told the broadcaster.
Kibatala said such comments amounted to interference and could affect the likelihood of a "fair trial".
Mbowe, who has been charged with terrorism financing and conspiracy, was due to appear in court on Friday amid tight security, but transport problems prevented him from doing so, Kibatala said.
His arrest came four months after Hassan took office following the sudden death of her predecessor John Magufuli.
There had been hopes Hassan would usher in change from the autocratic rule of Magufuli, nicknamed the "Bulldozer" for his uncompromising style.
But Chadema leaders say the arrests of Mbowe and his colleagues reflect a deepening slide into "dictatorship."
On Wednesday, the authorities suspended the newspaper owned by the ruling party for two weeks after it published a "false" story about Hassan, the first time such a ban was imposed on the publication.
Uhuru, the paper of Hassan's Chama Cha Mapinduzi (CCM) party, had published a front-page story claiming she was not considering running for the presidency in 2025.
A party official said several editors, including the paper's chief editor, had also been suspended.
Prosecutors say the terrorism charges against Mbowe do not relate to the constitutional reform forum Chadema had planned to hold in the northwestern port city of Mwanza last month, but to alleged offences last year in another part of Tanzania.