But, after the vote was counted and against Senate rules and procedures, Sen. Sani Ahmed Yerima opposed the age limit, saying it goes against Islamic law.
"By Islamic law any woman that is married, she is of age, so if you now say she is not of age then it means that you are going against Islamic law," declared Yerima.
In the second vote forced by Yerima, several Muslim senators who had voted "yes" to set the age at 18, changed their minds, and the amendment did not get the two-thirds majority needed to pass, according to Sen. Tenyi Abaribe, the Senate spokesman.
So now, a Nigerian male must be 18 to renounce his citizenship but a girl married at 14, like one of Yerima's wives, can do it at a much younger age.
The vote applied only to citizenship, Abaribe emphasized.
Nigerians are seeing it as a confrontation with other laws that dictate a girl should not marry until she is 18 and, coming from a man known for marrying children, as a gateway challenge to those laws.
"The government needs to stick with the age of consent being 18 and to work with communities in recognizing that a child is a child," said Iheoma Obibi of the Nigerian Feminist Forum.
Her forum has lodged a protest along with the Gender and Constitutional Reform Network, and other groups including advocates using the Twitter hash tag #ChildNOTBride.
Nobody knows quite how many thousands of girls are forced into marriage here some say more than 50 percent of girls in the Muslim north often sold for a bride price in mainly poor and rural communities.
Yerima is infamous for his marriage to a 14-year-old Egyptian girl in 2010, when he was 49. He had divorced a 17-year-old whom he married when she was 15, to comply with Islamic law that allows a maximum of four wives at a time.
Then U.S. Secretary of State Hillary Clinton rebuked Yerima for "child slavery," and the State Department's 2012 report on human rights complained that Nigerian authorities did nothing to prosecute Yerima.
In a public outcry at the time, activists demanded that prosecutors investigate and Yerima be forced out of Parliament. Instead, the National Agency for the Prohibition of Trafficking in Persons questioned Yerima but said it lacked evidence to charge him. It said Yerima had paid $100,000 for the young bride, the daughter of his Egyptian driver.
The government agency recommended the justice minister and attorney general investigate Yerima for violating Nigeria's 2003 Child Rights Act.
"But nothing was done, he has got away with it," said women's right activist Obibi.
She pointed to the horrendous statistics that show a high percentage of young girls suffer damage in child birth, with maternity wards in the mainly Muslim north filled with young mothers whose vaginas, uteruses and anal passages have ruptured. According to the U.N. Children's Fund, Nigeria has 2 percent of the world's underage marriages but 10 percent of its victims of vesicovaginal fistula, which leaves the girls incontinent, dripping urine and feces.
Such girls are often divorced and abandoned, left to beg on the streets or turn to prostitution, Obibi said.
Dupe Killa, a human resources manager and mother of two girls, started an online petition at #ChildNOTBride after the Senate vote. When the petition was oversubscribed within hours, she decided to take to the streets to get signatures and has won support, including from the country's influential Movement for Islamic Culture and Awareness.
Protesters handed out #ChildNOTBride flyers featuring the silhouette of a girl with pig tails and bows set in a protective red circle. The petition urges the Senate and National Assembly to stop "loopholes within which Nigeria can continue to discriminate against half the population (i.e. females)."
Yerima was instrumental in introducing Shariah law to Nigeria's nine northern states in 2000 and 2001, when he was governor of Zamfara state. Another three states where Muslims form a plurality have since instituted Shariah as a substitute for Western-style family law for Muslims wishing to use it. The other 25 states are governed by secular law.
Shariah is interpreted differently by scholars and laws differ according to a country's history and culture. Fani-Kayode, the former minister, said most Muslim countries have banned child marriage and rape.
Nigeria's population, at 160 million the biggest in Africa, is almost equally divided between a mainly Muslim north and majority Christian south.
While the religions coexist peacefully in most of the country, there are frequent and bloody clashes between militant Muslims and Christians in the north. Tens of thousands of been killed over the years, and churches and mosques razed.
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Associated Press photographer Sunday Alamba contributed to this report from Lagos.
