President Yahya Jammeh of Gambia, has been told to ignore the signing of the new anti-gay legislation.
The plea was made by Amnesty International and Human Rights Watch, HRW, which said the definition of "aggravated homosexuality" was vague in The Gambian bill.
Homosexual acts are already illegal in The Gambia, but MPs passed a bill on 25 August imposing life sentences for "aggravated homosexuality".
Jammeh has been known for his tough opposition to gay rights. He calls them "vermin". The President had threatened to behead them once.
Uganda's Constitutional Court struck out a similar law last month on the grounds that it was passed by MPs without a quorum.
Its ruling followed an outcry from rights groups and Western governments - US President Barack Obama described the legislation as "odious".
Among those who could be given the life sentence were "repeat offenders" and people living with HIV who are suspected to be gay or lesbian.
A person who had homosexual relations with a minor could also be convicted of "aggravated homosexuality", Reuters news agency reports.
"President Jammeh should not approve this profoundly damaging act that violates international human rights law," said Stephen Cockburn, Amnesty's deputy regional director for West and Central Africa.
Graeme Reid of HRW said it would if the bill was passed it will further add stigma on the already marginalised group.
Homosexual acts are already punishable by up to 14 years in prison in The Gambia.