The authorities in the Nigerian city of Maiduguri are blaming an Islamist group, Boko Haram, for an attack in which up to four people are reported to have died and around 37 injured.
Details of what happened are confused.
Some reports say a blast rocked an area near a mosque in the city just as people were leaving Friday prayers.
The reports quote witnesses as saying they had seen four bodies.
However, police said two people died in a shootout with security forces when armed robbers attacked a market near a mosque.
The BBC says Maiduguri has been plagued by attacks from Boko Haram, which wants to impose strict Sharia law across Nigeria.
The group has claimed responsibility for an attack on several churches in the central city of Jos on Christmas Day.
At least 42 people were killed in those attacks.
