A bomb blast at the entrance of the police headquarters of northeast Nigeria's Borno state has killed many people, mostly police, witnesses and security officials said.
A police officer at the scene, who could not be named, said five police vans had been loaded with the dead.
"Many people, mostly members of the police which include men and women were killed in the explosion," said witness Ali Alhaji in the city of Maiduguri, the focal point of an insurgency by Islamist sect Boko Haram.
Boko Haram, which wants to create an Islamic state in parts of Nigeria, has been blamed for hundreds of bomb and gun attacks on security forces and civilians over the past two years.
There was no immediate claim of responsibility for the bomb.
Police stations are a favourite target for the insurgency, which flared up partly in response to police brutality against its members, including its founder Mohammed Yusuf, killed in police custody during a crackdown in 2009.
From being a reclusive clerical movement opposed to Western education last decade, Boko Haram has radicalised and mushroomed to become the main security threat facing Africa's top energy producer, and has linked up with other Islamist groups in the region such as al Qaeda's north African wing.
It is based far from oil producing facilities in the south, although its fighters have successfully targeted the capital Abuja in the middle of the country a handful of times.
Nigerian forces shot dead 16 suspected militants in a fire fight with Islamist sect Boko Haram on Tuesday, the military said, the sort of move which sometimes provokes a retaliation from the Islamists.