The killing of Ally Kibao, a member of the secretariat of the main opposition CHADEMA party, may taint the reformist image of President Samia Suluhu Hassan, who has tried to ease repression since succeeding John Magafuli who died in office three years ago.
Kibao's body was found on Saturday morning, a day after two armed men removed him from a bus travelling from Dar es Salaam to the north-eastern port city of Tanga, CHADEMA chairman Freeman Mbowe told journalists late on Sunday.
"The (preliminary) post-mortem has been done and it is obvious that Ally Kibao has been killed after being severely beaten and even having acid poured on his face," Mbowe said, adding that a full autopsy report would be completed on Monday.
President Hassan said she had ordered an investigation into what she described as Kibao's assassination.
"Our country is democratic and every citizen has the right to live. The government I lead does not tolerate such brutal acts," she wrote on X on Sunday.
Police said in a statement they were investigating the "tragic incident".
Kibao's death comes a month after police
arrested and briefly detained more than 500 CHADEMA supporters, including their top leadership, as they attempted to gather for a meeting of the party's youth wing in the southwest of the country.
Hassan has taken some steps to ease restrictions on the media and opposition since coming to power, but rights groups say arbitrary detentions have continued.
Mbowe urged the president to form a judicial commission to investigate abductions and the killing of Kibao because, he said, police were among the suspects in the case.