India's laws on homosexuality threaten human rights and encourage the spread of HIV, a leading rights watchdog has told the prime minister in a letter.
India says same-sex relationships are "unnatural"
Human Rights Watch wrote to Manmohan Singh after police in the northern city of Lucknow allegedly carried out a sting operation on gay men.
It accused the police of "shameful" harassment. Police said those arrested had engaged in "unnatural acts".
Homosexuality is illegal in India and can carry a 10-year sentence.
Human Rights Watch says that last week police officers in Lucknow posed as gays on a website, entrapping one man and forcing him to call others who were then arrested.
Scott Long, director of Human Rights Watch's Lesbian, Gay, Bisexual and Transgender Rights Programme, said: "Lucknow police have a shameful record of harassing gay men as well as non-governmental organisations that work with them.
"They are able to do so because India's government clings to the criminalisation of homosexual conduct, which only prevents people from coming forward for HIV/Aids testing, information and services."
The United Nations' Aids body, UNAids, also condemned the arrests.
Denis Broun, UNAids India coordinator, said: "Criminalisation of people most at risk of HIV infection may increase stigma and discrimination, ultimately fuelling the Aids epidemic."
Lucknow police spokesman Ashutosh Pandey told Reuters those arrested had "established online internet links with gay groups outside the country too" and would not be released.
The 145-year-old colonial Indian Penal Code clearly describes a same-sex relationship as an "unnatural offence".
Many people in conservative India regard same-sex relationships as illegal or even blasphemous.
In 2004, the Indian government opposed a legal petition that sought to legalise homosexuality - a petition the high court in Delhi dismissed.
The government argued that the abolition of the law dealing with what it termed as "unnatural sex acts" could result in an increase in delinquent behaviour.
"While the right to respect for private and family life is undisputed, interference by public authority in the interest of public safety and protection of health and morals is equally permissible. This is precisely what the law does," said a government affidavit.