Lagos - Gay activists on Tuesday vowed to fight the government after the country outlawed same-sex unions, warning the act legitimised homophobic violence and increased fears of persecution.
The United Nations also attacked the Same Sex Marriage (Prohibition) Bill 2013, accusing President Goodluck Jonathan of trampling over a raft of basic human rights and threatening vital healthcare plans.
The presidency has dismissed international concerns about the law, which threatens up to 14 years in jail for anyone in a same-sex marriage or civil partnership, saying it reflects public opinion.
The country's newspapers largely ignored the development, while there was wide support on social media, reflecting deeply held religious views across the west African nation.
Evangelicalism Christianity is popular in Nigeria's south, while the north is mainly Muslim and Islamic law runs alongside state and federal justice systems.
Research published in June last year by Pew Research Global Attitudes Project suggested that 98 percent of Nigerians thought homosexuality should not be accepted by society.
The law, however, has spread fear among Nigeria's lesbian, gay, bisexual and transgender (LGBT) community and disappointment among their supporters abroad.
One gay man in Lagos, whom AFP did not identify in case of reprisals, said he was "devastated" by Jonathan's actions and "terrified" at the consequences.
"I am concerned especially for LGBT people up north, in the (southern) Niger Delta... and for LGBT people in the whole of the country actually," he added.
"They (the government) have just legalised violence, stigma and discrimination.... Our situation has gone from bad to worse."
In London, LGBT activist and human rights campaigner Davis Mac-Iyalla said the legislation would force the community deeper underground and create a damaging climate of fear and suspicion.
Included in the law is a provision to punish with a jail term of up to 10 years anyone who registers, operates or participates in gay clubs, societies and organisations.
Anyone who directly or indirectly makes a public show of a same-sex relationship will also break the law and could face jail. 'A licence to violence'
UNAIDS and the Global Fund to fight AIDS, Tuberculosis and Malaria said they feared that LGBT people in Nigeria could be denied vital health services.
Nigeria has the second-largest HIV epidemic globally, with an estimated 3.4 million people living with HIV in 2012.
UNAIDS and the Global Fund called in a statement for an "urgent review of the constitutionality of the law in light of the serious public health and human rights implications".
Mac-Iyalla said the law was not about gay marriage, which was not a pressing issue in Nigeria, but "the further punishment and criminalisation of homosexuality".
Nigeria is one of 78 countries worldwide, many of them in Africa, where homosexuality is illegal, according to UNAIDS.
Nevertheless, he added: "I still believe there are lawyers and people from the human rights angle would be interested in taking this case.
"Human rights can be fought from anywhere, nationally or internationally. We're not going to keep quiet and accept such a draconian law."
The US Secretary of State, John Kerry, led international condemnation of Nigeria, saying it was "deeply concerned" about the law, which follows similar legislation proposed in Uganda.
Amnesty International and gay rights activists and groups have also condemned the law, calling it discriminatory and incompatible with international human rights laws to which Nigeria is a signatory.
The UN human rights commissioner, Navi Pillay, said the law "makes an already-bad situation much worse", reinforcing existing prejudices and having the potential to cause an increase in violence.
"Rarely have I seen a piece of legislation that in so few paragraphs directly violates so many basic, universal human rights," she added.
"Rights to privacy and non-discrimination, rights to freedom of expression, association and assembly, rights to freedom from arbitrary arrest and detention: this law undermines all of them."
The gay Nigerian man spoken to by AFP agreed: "It (the law) is a licence to violence.
"In this country we have people being lynched because of their sexual orientation and gender identity...
"I am afraid for people who can't even speak for themselves and who are going to end up in jail without anyone knowing about it."