A proposed Nigerian law banning same-sex marriages is a threat to democracy, says Human Rights Watch.
Parliamentary insiders say the bill is likely to be passed
Writing to the Nigerian Senate, they said the legislation, "contravenes the basic rights to freedom of expression, conscience, association, and assembly".
The rights group urges the Nigerian National Assembly to reject the bill.
If the proposed law is approved, anyone who speaks out or forms a group supporting gay and lesbian rights could be imprisoned.
The bill has divided both chambers of the Nigerian parliament as some MPs see legislation as a move to save Nigerian morals and cultural values.
Others legislators who reject it say it say it is anti-freedom and portrays Nigeria's democracy in bad light.
The proposed bill has also created divisions among Nigerian civil society organisations.
"Supporters of the same-sex marriage in Nigeria don't know what they are saying. As far as we are concerned, gay marriage is not allowed in Africa," says Emmanuel Onwubiko, a senior commissioner at Nigeria's Human Rights Commission.
Nigeria's mainstream churches back the anti-gay bill
"If South Africa want to do it, that is their business. It is not Nigerian to be gay, let alone going ahead to legally get married as gay and even live as a family with adopted children. It's completely alien to our culture," he told the BBC website.
But Human Rights Watch says the proposed law "would infringe against the right to privacy, and would strike at the fundamental freedoms enjoyed by all individuals in Nigeria's long-vigorous civil society."
The New York-based group also says the bill is being "pushed forward with a secretive and undue haste apparently designed both to secure its passage before upcoming elections and to foreclose full debate on its provisions."
Parliamentary insiders say the bill is likely to be passed by both chambers of the Nigerian National Assembly by the end of March.
Nigeria, like many African countries, is a conservative society where homosexuality is considered a taboo.
The proposed legislation which was introduced in the Nigerian parliament by President Olusegun Obasanjo early last year has already passed first and second readings in both chambers of the National Assembly.