The bomber drove a jeep right inside the packed St Rita's church, in the Malali area of Kaduna, a volatile ethnically and religiously mixed city, in the morning and many people were wounded, several witnesses said.
"I cannot tell you how many casualties, but there were many. The heavy explosion also damaged so many buildings around the area," said survivor Linus Lighthouse, saying he thought there had been two explosions in different parts of the church.
Other witnesses and the police said there had just been one bomber however.
There was no immediate claim of responsibility but Islamist sect Boko Haram has claimed similar attacks in the past and has attacked several churches with bombs and guns since it intensified its campaign against Christians in the past year.
One wall of the church was blasted open and scorched black, with debris lying around. Police later moved in and cordoned the area off.
Shortly after the blast, angry Christian youths took to the streets armed with sticks and knives. A Reuters reporter saw two bodies on the roadside lying in pools of blood.
"We killed them and we'll do more," shouted a youth, with blood on his shirt, before police chased him and his cohorts away. Police set up roadblocks and patrols across town in an effort to prevent the violence spreading.
Another witness, Daniel Kazah, a member of the Catholic cadets in the church, said he had seen three bodies on the bloodied church floor after the bomb. "But still others were taken to the mortuary," he said.
An emergency worker on the scene, who had helped move casualties but was not authorised to give his name, estimated the total number of dead and wounded at around 30.
A spokesman for St Gerard's Catholic hospital, Sunday John, said the hospital was treating 14 wounded but had not received any dead.
Islamist sect Boko Haram is fighting to try to create an Islamic state in Nigeria, whose 160-million people are split roughly evenly between Christians and Muslims.
Some of the attacks on churches have seemed calculated to stir sectarian tensions along Nigeria's volatile middle belt, where its largely Christian south and mostly Muslim north meet.
Kaduna lies along that fault line, and many of its neighbourhoods are mixed.
A bomb in a church in Kaduna state in June triggered a week of sectarian violence that killed at least 90 people. – Reuters