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 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 released on bail on Thursday.
Last week Ester Bulaya, another Chadema lawmaker, was arrested in the north of the country for planning a meeting outside of her electoral constituency.
"What law prohibits that?" asked the LHRC statement.
On Wednesday, another MP, Godbless Lema was arrested for taking seven minutes longer than his allocated time to address 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.
Chadema's youth wing on Thursday said in a statement they had written to police chief Simon Sirro giving him seven days to tell them "whether or not being a member of an opposition party is a crime".
If there is no response they plan a countrywide protest on August 31.