The US Embassy and British High Commission in Tanzania have jointly raised concerns over the violation of human rights.
They said frequent lengthy pretrial detentions and shifting of charges are evidence of injustices.
The joint statement released on Friday also questioned how Tanzanian top investigative journalist Erick Kabendera was handled.
They said Kabendera was denied access to his lawyer in the early stages of his detention, which is contrary to Criminal Procedures Act.
The two urged the Tanzania government to guarantee due process to its citizens, which is a basic human right.
Kabendera was forcefully arrested by Tanzanian police on Monday 29 for failing to present himself for questioning.
Dar es Salaam police chief Lazaro Mambosasa, said Kabendera had refused to comply with a letter asking him to report to police.
He was detained for a week before being charged with "economic crimes".
The journalist was not asked to enter a plea and as the charges are considered too serious for him to be granted bail, he will be remain in custody until his case is heard.
