At least seven people have been killed and dozens injured in a suicide bombing during Mass at a Catholic church in northern Nigeria, officials say.
An explosive-laden vehicle drove into the church and detonated its load, ripping a hole in the wall and roof.
The attack happened in Kaduna, which has been targeted by Islamist militant group Boko Haram in the recent past.
President Goodluck Jonathan promised to "redouble" his government's efforts to tackle terrorism and violence.
He called the attack part of an "unfortunate and unacceptable trend that threatens the peace and stability of our nation".
A spokesman for the local governor has called for calm, pleading with people on local radio not to retaliate.
The Nigerian Emergency Management Agency told the BBC that Christian youths attacked a vehicle that had come to rescue survivors after the attack, smashing one of the windows.
Unconfirmed reports said at least two people were killed in reprisal attacks by Christians after the bombing.
No group has said it carried out the bombing.
Looking for sanctuary
The attack happened at St Rita's church in the Malali neighbourhood of the city.
The vehicle had been stopped at the security gate outside the church.
The driver initially reversed, but then careered straight through the church wall and detonated the bomb.
Members of the choir are thought to be among the dead and injured.
The church was surrounded by soldiers and police after the blast, and ambulances were taking the injured to hospital.
Nigeria's north has a large Muslim majority whereas the south is most populated by Christians and those who follow traditional religions.
Kaduna is on the dividing line between the two areas.
The BBC's Will Ross in Kaduna says many people have come to the city in recent months in search of sanctuary from violence in other parts of northern Nigeria.
Boko Haram is fighting to overthrow the government and impose an extreme form of Islamic law.
The group has said it carried out previous attacks on churches in Kaduna state in June.
At least 50 people were killed in the bombings and the reprisals that followed.