GENEVA (Reuters) - A legal ban on same-sex marriage in Nigeria, Africa's largest country, violates international accords and could bring mob law against gays onto its streets, the United Nations' human rights chief Navi Pillay has said.

The comments in the Nigerian capital Abuja from the outspoken Pillay, a former top judge in South Africa, came as in Geneva Russia and Islamic countries urged more global focus on protecting traditional family life.

Pillay told her largely Nigerian audience on Feb 14 that the country's lesbian, gay, bisexual and transgender (LGBT) community was "living in fear", since the loosely-drafted law, not yet put into effect, had gone onto the statute books.

"The law violates international law in that it is discriminatory and seriously impinges on the freedom of expression and freedom of assembly," she declared.

It could also "lead to human rights defenders advocating for the rights of LGBT people receiving draconian prison sentences," Pillay said, in a clear reference to phrasing in the measure that bars promotion of homosexuality.

U.N. human rights officials were already getting reports of widespread arrests of gays and lesbians in some parts of Nigeria, of physical attacks "including by mobs," and a rise in blackmail and extortion, the U.N. rights chief said.

DEFENDING THE FAMILY

Officials in Nigeria, and other African countries like Uganda, whose own legislation criminalising homosexuality has also been criticised by Pillay, often argue that the measures are aimed at defending family life from attack.

At the U.N.'s 47-nation Human Rights Council, Russia -- whose own restrictions on gays have brought strong criticism from Western and some other countries -- argued on Friday that the family was the "natural and basic unit of society."

Russia has recently introduced a law banning "promotion of homosexuality", a move that led Western leaders to stay away from the recent Winter Olympics and the Paralympic Games in the southern Russian resort of Sochi

Reflecting the stand of the Russian Orthodox Church, with which President Vladimir Putin identifies, a Moscow diplomat told the Council that it should be addressing the "problems facing the institution of the modern family."

A similar view was voiced by the 57-member Organisation of Islamic Cooperation (OIC), which fiercely rejects the idea that gays are covered under U.N. rights pacts, and by Namibia.

But Netherlands and the international humanist organisation IHEU rejected this stance. A Dutch diplomat told the council that the judicial harassment of defenders of the LGBT community and criminalisation of gays were deplorable.

"Imprisoning people on the basis of sexual orientation is a violation of human rights, including the freedom of expression, association and assembly," an IHEU speaker from the British Humanist Association said. (Reported by Robert Evans; Editing by Tom Heneghan)