The group were arrested when police officers raided a party at Pongwe Beach on Saturday night, according to Amnesty International.
Six others at the event fled.
Amnesty International said that the men were arrested for allegedly carrying out a gay marriage, with police saying that they found the men sitting in pairs “two by two”.
“This is a shocking blow following the Tanzanian government’s assurance that no one would be targeted and arrested because of their actual or perceived sexual orientation and gender identity,” said Seif Magango, Amnesty International’s deputy director for East Africa.
“This appalling attack on Tanzanian people simply exercising their human rights shows the danger of inflammatory and discriminatory rhetoric at senior levels of government.
“We now fear these men may be subjected to forced anal examination, the government’s method of choice for ‘proving’ same-sex sexual activity among men.
“This must not be allowed to happen – these men must be released immediately.”
The arrested men are currently being held at Chakwal police station on the island of Unguja, despite no charges having been brought against them.
Fears are rising about the treatment of LGBT+ people in Tanzania after the regional commissioner of the country’s economic capital Dar es Salaam called on the public to report people suspected of being gay.
“Give me their names,” Paul Makonda said, speaking on 29 October.
“My ad hoc team will begin to get their hands on them.”
Mr Makonda also announced plans to set up a special committee to identify and punish gay people, prostitutes and online fraudsters.
The committee will scour the internet to identify videos featuring supposedly gay content and Mr Makonda warned citizens to delete any “sex pictures” they had stored on their phones or face arrest.
He added that thousands of messages had already been sent in expressing support for the policy and outing people believed to be gay.
The Tanzanian government subsequently distanced itself from Mr Makonda’s comments and called them “personal opinion”.
However, LGBT+ people have criticised the ruling administration for its slow response and for failing to condemn the city chief.
In response, the European Union recalled its ambassador to Tanzania over “the deterioration of human rights and rule of law” in the East African nation.
More about: Tanzania