The Islamist militant group Boko Haram claimed responsibility on Monday for the abduction of more than 200 schoolgirls during a raid in the village of Chibok in northeast Nigeria last month, the French news agency AFP reported, citing a video it had obtained.
"I abducted your girls," Boko Haram leader Abubakar Shekau said in the video, according to AFP. It did not immediately give further details.
Meanwhile, Nigerian authorities have arrested a leader of a protest last week in the capital Abuja that called on them to do more to find the schoolgirls, a presidency source and another organizer of the protest said on Monday.
Boko Haram insurgents, who want to carve an Islamic state out of Nigeria, stormed a secondary school in the village of Chibok, northeast Nigeria, on April 14. They carted off the girls in trucks and nothing has been heard of them since.
The police spokesman did not immediately respond to a request for comment on the arrest, but the presidency source said Naomi Mutah Nyadar had been detained for allegedly and falsely claiming to be the mother of one of the missing girls.
Nyadar was picked up late on Sunday after a meeting she and other campaigners had held with President Goodluck Jonathan's wife, Patience, concerning the girls.
She was taken to Asokoro police station, near the presidential villa, said fellow protester Lawan Abana, whose two nieces are among the abductees.
"Ms Naomi was arrested yesterday evening," he told Reuters by telephone. "We are begging them to save our daughters. Instead of taking steps to rescue them they are jailing us."
The presidency source said: "(Nyadar) was arrested because of impersonation. She claimed that she was one of the girls' mothers, so she's just being questioned by the police."
Abana denied Nyadar had made any such claim.
"They are claiming it is a hoax and that her daughter was not abducted. But when we say 'bring back our daughters' the campaign means it in the broader sense of 'daughters of Nigeria'," Abana said. "They are so clueless."
Nigeria's government is becoming increasingly nervous about security for the World Economic Forum (WEF) for Africa, an annual gathering of the rich and powerful to be held in Abuja this week for the first time.
The girls' abductions have been hugely embarrassing for the government ahead of the WEF, which was supposed to focus attention on the growth potential of Africa's biggest economy but threatens to be overshadowed by the girls and by Nigeria's mounting security woes.
In a televised "media chat" late on Sunday, President Jonathan pledged that the girls would soon be found and released, but he also admitted he had no idea where they were.