LAGOS (Reuters) - A Nigerian rights group has launched a petition to stop plans by religious leaders and a state lawmaker to push 100 girls and young women into marriage in a mass ceremony next week, which have sparked outrage in the West African nation.

The plans, first revealed by Abdulmalik Sarkindaji, speaker of the national assembly in the largely Muslim northern state of Niger, were criticised by Nigeria's women's affairs minister Uju Kennedy-Ohanenye, who said she would seek a court injunction to stop the ceremony and establish if any of the girls were minors.

Sarkindaji said the girls and young women were orphans whose parents were killed in attacks by kidnapping gangs that roam northern Nigeria. He pledged to pay dowries to the bridegrooms.

The rights group Concerned Nigerian Citizens, in a petition launched on Wednesday that has garnered over 7,000 signatures, said the Niger state government should prioritise the education of the girls instead of forcing them into marriage.

"We demand immediate action to halt the proposed forced marriages and to instead implement measures that will empower these girls to lead dignified and fulfilling lives," the group said.

But Sarkindaji and the Imams Forum of Niger said the marriage ceremony would go ahead on May 24.

The ages of the girls were not immediately known but the lawmaker and the Imams say they are not under age. Reuters was not able to identify or speak to any of the girls and young women.

Child marriages are common in the mostly Muslim north, where poverty levels are higher than the largely Christian south. Although the legal age of marriage is 18 under federal law, Nigerian states can set their own age.

Niger's legal marriage age is also 18, Sarkindaji's spokesperson Auwal Mohammed said, but added that under Sharia law, which is practiced in the state, a girl can be married upon reaching puberty.

After meeting on Wednesday, the Imams Forum said it would take legal action against the women's affairs minister if she did not withdraw her statement suggesting the girls were minors, its secretary Umar-Faruk Abdullahi said on local TV.

"We have given the minister seven days to withdraw her statement she used against us, against our speaker, against the Muslim community .. that we want to force them into marriage and the children are under age," said Abdullahi.

Kennedy-Ohanenye did not respond to calls and messages to her phone.

(Reporting by MacDonald Dzirutwe and Ahmed Kingimi in Maiduguri, Editing by Ros Russell)