Tanzania's main human rights group on Thursday criticised the government for its "harassment" of opposition lawmakers, three of whom have been arrested over the past week.
In a statement, the Legal and Human Rights Centre (LHRC) said the string of arrests amounted to a "violation of political and civic rights" in a country where multi-party politics is enshrined in the constitution.
On Tuesday, opposition MP Tundu Lissu was arrested for allegedly insulting President John Magufuli after revealing that Canada had seized a commercial aircraft purchased by the government over an unpaid $38-million (32-million-euro) debt to a Canadian engineering company.
It is the sixth time Lissu, a lawyer and deputy head of the main opposition Chadema party, has been arrested this year. In July he was charged with hate speech after calling Magufuli a "dictator".
He was arrested on leaving court after attending a hearing in one of the other cases against him.
Three days earlier, Ester Bulaya, another Chadema lawmaker, was arrested in the north of the country before taking part in a meeting outside of her electoral constituency.
"What law prohibits that?" asked the LHRC statement.
And on Wednesday, another MP called Godbless Lema was arrested for taking seven minutes longer than his allocated time for addressing a rally.
The LHRC urged the government to respect the right of all political parties to freely carry out their activities.
"Being the member of a different party (than the ruling party) is not a crime."
Magufuli, who came to power in 2015 as a corruption-fighting "man of the people", has been increasingly criticised over his authoritarian leadership style, with a clampdown on the opposition, journalists and artists.