Eyewitnesses in the northern Nigerian city of Kano say they have counted more than 20 bodies after co-ordinated bomb attacks on Friday.
A 24-hour curfew is in place in the city. Police have confirmed seven deaths in various locations.
Police stations and the state police HQ were among the targets, and gunfire was heard across the second biggest city.
The militant Islamist group Boko Haram, whose name means "Western education is forbidden", said it was responsible.
The group has been behind a recent campaign of violence in the mainly Muslim north.
Meanwhile, organisers of a controversial civil activists' mass rally set for Saturday in the commercial capital Lagos called off the event in light of the attacks.
Organisers of the demonstration against government corruption and the military's presence in Lagos say they fear their protest could be infiltrated by militants sent to cause mayhem and cost more lives.
Nervous crowds
In a statement, police said "seven casualties have been confirmed from different locations of the attacks" in Kano, amid fears that the death toll could rise further.
But eyewitnesses said they had seen bodies littering the street and being loaded into wagons. More than 20 were counted.
The police statement said four police stations around the city, the headquarters of the State Security Service (SSS), as well as passport and immigration offices were targeted.
The BBC's Mark Doyle, in the outskirts of Kano, says he has seen one police station with its roof completely burnt off, though it was not clear whether this was caused directly by an explosion or by fire.
The atmosphere there is nervous, and a large crowd outside the police station quickly dispersed when soldiers arrived, our correspondent adds.
He says this is Boko Haram's most serious attack on the police and is deeply embarrassing for the authorities.
The curfew would be in place in Kano until further notice, officials said.
The wounded were reported to include foreigners from an area near the SSS headquarters, where many expatriates - particularly Lebanese and Indians - live.
There was also a shoot-out at the headquarters of the state police in the city's eastern district of Bompai, reports said.
One local man, Andrew Samuel, described the scene of one blast: "I was on the roadside and I just heard a 'boom'. As I came back, I saw the building of the police headquarters crashing down and I ran for my life."
Witnesses said the bomber who attacked one of the police stations pulled up outside the building on a motorbike, dismounted and ran inside holding a bag.
Nigeria's Channels TV said one of its reporters, Enenche Akogwu, had been killed in the attacks.
It said he had been "shot by unknown gunmen suspected to be members of the Boko Haram sect", outside the state government house.
Boko Haram later claimed responsibility for 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 group 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.
The death of the Boko Haram leader Muhammed Yusuf whilst being held by police in 2009 is also often cited as the reason for attacks on state institutions by the group, the BBC's Mark Lobel in Lagos reports.
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.
