Karaua has dubbed the move a sign that the Tanzanian authorities would not give a fair trial to Chadema party leader Tindu Lissu.

Lissu, who Karua says is the main challenger to Tanzania's President Samia Suluhu Hassa, could be facing the death penalty for his alleged crimes.

Lissu's Chadema party has been disqualified from presidential and legislative elections due in October, after it refused to sign an electoral code of conduct. 

Karua says Tanzania is seeing a "total erosion of democratic principles.”

The former justice minister has been vocal about "democratic backsliding" in East Africa.