A series of explosions has rocked Kano, the biggest city in northern Nigeria.
One of the blasts killed two police officers at the federal police headquarters in the city, AFP reports. Attacks have been reported on at least three other police stations.
The authorities in Kano state have imposed an immediate 24-hour curfew.
The Islamist militant group Boko Haram has claimed responsibility. The group has been behind a recent campaign of violence in the mainly Muslim north.
The BBC's Yusuf Yakasai in Kano says there was panic in the city as plumes of smoke rose into the sky.
Our reporter says that besides the police headquarters in the west of the city, police stations in near the centre and the south were targeted.
There has also been a shoot-out at another important police building in the east of the city.
A resident near the city centre told the BBC that he had seen bodies being carried out of a police station near the city centre, but did not know if they were injured or dead.
A reporter for the AP news agency said one of the explosions was powerful enough to shake his car several miles away.
Claim of responsibility
Boko Haram, whose name means "Western education is forbidden", has said it carried out the attacks.
A spokesman for the group, Abul Qaqa, told journalists in the north-eastern city of Maiduguri, the group's base, that it had carried out the attacks because the authorities had refused to release members arrested in Kano.
The group wants to establish Islamic law in Nigeria. It started to stage drive-by shootings in 2010 on government targets in Maiduguri.
Analysts say this is one of Boko Haram's largest simultaneous attacks, and certainly its largest assault on Kano.
It stepped up its attacks in 2011, targeting police headquarters and the UN in the capital Abuja.
In recent weeks, southerners, who are mostly Christians or animists, living in the north have been the targets of deadly attacks.