Daniel John was responsible for the Chadema party in Hananasif district in Dar es Salaam.
Chadema accuses supporters of the ruling party CCM of being behind the attack.
A local leader of the main Tanzanian opposition party has been found beaten to death after men bundled him into a car, party chairman Freeman Mbowe said Wednesday.
Daniel John was responsible for the Chadema party in Hananasif district, Dar es Salaam, where a heated local election campaign is under way.
"Daniel John was kidnapped on Monday in the street by men in a car," Freeman Mbowe told AFP, the leader of the opposition in parliament.
A friend of John, who was snatched at the same time but who survived the attack after being left for dead, was able to describe what then happened.
"They were beaten and tortured, and the torturers were asking why they were campaigning for Chadema's candidate," Mbowe said.
John's body was later delivered to a hospital, with machete wounds to the head, said Mbowe, who accused supporters of the ruling Chama Cha Mapinduzi (CCM) party of being behind the attack.
"The party in power wants to get everything by force," said Mbowe.
He was rushed to Nairobi where he was in intensive care for several months, before being transferred to a hospital in Brussels.
Chadema accuses the ruling party of being behind that attack, claims rejected by the government.
Tanzanian President John 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.