Amnesty International said Thursday that plans by Tanzania's economic capital to hunt down and arrest suspected homosexuals were leading it down a "dangerous path".
Dar es Salaam governor Paul Makonda on Monday urged citizens to begin reporting homosexuals for round-ups to begin next week in the country, where anti-gay rhetoric has soared in recent years.
"It is extremely regrettable that Tanzania has chosen to take such a dangerous path in its handling of an already marginalised group of people," said Joan Nanyuki, Amnesty's regional director.
"The idea of this taskforce must be immediately abandoned as it only serves to incite hatred among members of the public."
Makonda, a fervent Christian and loyal ally of President John Magufuli, said on Monday he expected criticism from outsiders for his hardline stance.
"I prefer to anger those countries than to anger God," he told reporters.
Makonda said homosexual behaviour "tramples on the moral values of Tanzanians and our two Christian and Muslim religions".
Anti-homosexual sentiment is rife in Tanzania, forcing most gays, lesbians and other sexual minorities to live in secrecy.
Political rhetoric against homosexuality has increased since Magufuli's 2015 election.
Last year the president said everybody should condemn homosexuality, "even cows", and soon after his government threatened to arrest or deport gay rights activists.
Three South Africans were subsequently expelled for allegedly advocating for same-sex marriage.
AIDS clinics have also been shut down under Magufuli, accused of "promoting" homosexuality, while he has encouraged women to abandon birth control and have more babies.
Under British colonial-era laws homosexuality is illegal in Tanzania, with same-sex acts between men punishable by a maximum life sentence.