The United States said Wednesday it was "appalled" by reports that Tanzanian police used excessive force when it rounded up several opposition members over a banned youth rally this week.
Leaders of the main opposition party Chadema, including chairman Freeman Mbowe, his deputy Tundu Lissu and their supporters, were arrested ahead of Monday's planned youth event in the southwestern city of Mbeya.
Rights groups and government opponents have criticised the arrests, saying they fear it could signal a return to efforts to intimidate the opposition ahead of national elections next year.
"We are appalled by credible reports that police used excessive police force during recent arrests," the US embassy in Tanzania said in a statement.
"The injuries sustained resulted in the hospitalisation of some opposition party members."
As many as 520 people were arrested across the country, police said, claiming that violence was planned.
Chadema said its top leaders had been beaten during their arrests and accused the police of torture.
"Police used electric gadgets to torture people during the arrest," Chadema Secretary General John Mnyika said on Wednesday.
The party said it will launch a lawsuit against the assistant registrar of political parties Sisty Nyahoza and police commissioner of operations Awadhi Haji.
"These two must be taken to court. We cannot issue a blanket condemnation of the police force as there are good people in the police," Mbowe said.
The arrests came despite President Samia Suluhu Hassan vowing a return to "competitive politics" and easing some restrictions on the opposition and the media, including the January 2023 lifting of a six-year ban on opposition gatherings.
She has sought to break with some of the authoritarian policies of her predecessor John Magufuli who died suddenly in March 2021.
The move prompted US Vice President Kamala Harris to hail Samia as a "champion" of democracy during a trip to Tanzania in March last year.
Tanzania's 2025 presidential and parliamentary elections will be the first since the death of Magufuli, who was nicknamed the "Bulldozer" for his authoritarian policies.
His presidency from 2015 to 2021 was marked by crackdowns on the press, freedom of speech and political opposition.
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"We echo the calls of Tanzanian civil society organisations and urge the government of Tanzania to uphold its constitution, and ensure a safe and open political space where all voices are free to participate," the US embassy said Wednesday.