The police estimate differed from the local hospital's estimate, which counted 51 people with blast injuries.
The blast happened at the main entrance of the ever-crowded Monday Market, the largest in the city of Maiduguri. About 12:40 p.m. Saturday, the girl was stopped for screening by vigilantes who man the gates of the market, according to witnesses and rescuers.
The girl was among the dead, said Gideon Jubril, Borno state's police spokesman. Her precise age was not immediately clear.
A source with inside knowledge of security operations described her as being about 17 or 18. Abur Babakura, who was at the scene, said she appeared to be about 10.
The explosives detonated as the girl was being screened with a metal detector at the market's entrance, he said. It's standard procedure for people to be screened before entering the market.
Although no one has claimed responsibility, Boko Haram militants are the main suspects.
This was the fourth attack on the city's market since July.
"The metal detector beeped indicating she was carrying a metal under her dress, but it exploded before she could be singled out from the crowd," said Babakura.
"We believe that ... an accomplice watching from a distance pressed the remote," he said.
Two of the vigilantes manning the entrance were among the dead, Babakura said. The source reported that at least six vigilantes were killed.
Many of wounded victims taken to Maiduguri Specialist Hospital have life-threatening injuries, said Musa Kyarimi, a nurse there.
The marketplace in Maiduguri has been a frequent target. More than 45 people were killed in late November in an attack, and more than 10 others died in another blast a week later, on December 1. Another bombing on July 1 left at least 15 people dead and several injured.
On Sunday, three people were killed and 43 injured in a separate attack in Potiskum, police said.
"Two female suicide bombers targeted the mobile phone market in Potiskum this afternoon, but they blew up before the could reach their targets," Marcus Danladi, police commissioner for Yobe state, said from the state capital of Damaturu.
The two bombers were also killed.
Boko Haram has terrorized
northern Nigeria since 2009, attacking police, schools, churches and civilians, and bombing government buildings. It also has kidnapped students, including more than 200 schoolgirls who were abducted in April and remain missing.
According to the source, the group is attacking villages, killing older men and women and rounding up the young to "do their dirty work," or to be used as bombers.
Maiduguri is the capital of Borno state. The deadly Saturday blast came after a bloody massacre in Baga, a fishing town about 125 miles away, where Boko Haram militants killed hundreds, possibly as many as 2,000 people.
That massacre has been described as Boko Haram's deadliest act by Amnesty International.
The Islamist group, whose name translates to "Western education is sin," has said its aim is to impose a stricter form of Sharia law across Nigeria, which is split between a majority Muslim north and a mostly Christian south.