Arrests made after Nigeria's President Goodluck Jonathan signed anti-gay bill. REUTERS

Police in Nigeria have started arresting gay men after making homosexuality illegal with a bill that bans same-sex marriage and civil unions.

The Same Sex Marriage Bill 2013 means that anyone in a gay marriage or relationship faces a jail term of up to 14 years. It also bans LGBT organisations.

Authorities in Nigeria began arresting gay men in the mainly Muslim north of the country. Human rights groups have warned that suspects would be targeted by anti-gay activists.

Dorothy Aken'Ova, executive director of the country's International Centre for Reproductive Health and Sexual Rights, said there had been mass arrests. She said police in Bauchi state had a list of 168 gay men, of whom 38 had been arrested.

However, Mustapha Baba Ilela, chairman of Bauchi state Shariah Commission, told the Associated Press that just 11 men had been arrested. He also denied any of them had been tortured.

He said all 11 – including 10 Muslims and one non-Muslim – had signed confessions saying they belonged to a gay organisation. However, some of them later retracted their statements after being officially charged.

Nigerian president Goodluck Jonathan signed the law banning gay marriage on 7 January.

Nigeria has become the 38th African nation that has enacted anti-gay legislation.

Reuben Abati, presidential spokesman, said Nigerians were happy about the new law: "This is a law that is in line with the people's cultural and religious inclination. More than 90% of us oppose same-sex marriage, so it is a law that is a reflection of the beliefs and orientation of Nigerian people. Nigerians are pleased with it."

The Same Sex Marriage Bill follows legislation in Nigeria that makes homosexual sex illegal.

The bill also prohibits gay people from meeting in groups of two or more and criminalises gay clubs and events.

It follows similar legislation that passed in Uganda at the end of last year, which extends the penalty of life imprisonment for anal sex to all acts performed by a same-sex couple, including kissing.