The body of Tapfumanei Masaya, a pastor and an activist with the Citizens Coalition for Change, CCC, was discovered Monday, 30 kilometers (18 miles) outside Harare, the U.N. rights office said.
He and fellow activist Jeffrey Kalosi had been snatched by armed men in broad daylight two days earlier while campaigning in the Zimbabwean capital for a CCC candidate.
"Both were reportedly tortured," U.N. rights office spokeswoman Liz Throssell said, adding that Kalosi had been released in the same area where Masaya's body turned up.
The killing follows a string of reported abductions, arrests and other violent acts that the CCC says are part of a campaign of intimidation against its supporters following disputed general elections last August.
Throssell noted that Zimbabwean authorities had said Masaya's killing was being investigated.
"We urge them to ensure there are thorough, prompt and independent investigations not only into his death but also into all allegations of people being tortured and kidnapped," she said.
"Perpetrators should be held accountable in fair trials that follow due process."
Throssell warned that crimes like Masaya's killing "violate not only the right to life, but also have a stifling effect on the rights to freedom of opinion and expression, freedom of association and equal participation."
In August, CCC leader Nelson Chamisa, 45, lost his bid for the presidency to incumbent Emmerson Mnangagwa, 81, in elections that also gave the ruling ZANU-PF party a majority in parliament.
International observers said the vote fell short of democratic standards.
Dozens of CCC lawmakers have lost their seats recently after what they say is an impostor posing as a party official recalled them, and parliament went along with it.
The move has triggered by-elections to be held on December 9 that could hand ZANU-PF, in power since independence from Britain in 1980, a two-thirds majority needed to amend the constitution.
Analysts believe the party might use a super majority to remove a two-term presidential limit and enable Mnangagwa, who came to power on the back of a coup that ousted long-time ruler Robert Mugabe in 2017, to rule beyond 2028.
