Suicide bombers targeted four churches in a series of attacks in northern Nigeria on Sunday, killing at least 25 people, many of them children, and prompting reprisal attacks.
As many as 80 people were also reported wounded in the restive Nigeria region that has become a centre of ethnic strife in the country.
In Kaduna city, where 10 people were killed inside a Catholic church and 29 wounded, the attacks also led to reprisals by Christian youth against local Muslims.
Following reports of violence, the government imposed a 24-hour curfew to prevent the situation from going out of hand.
In all, two churches were attacked in Kaduna city while another two were bombed in a nearby Zaria city. Red Cross officials in the city said in a church located in Zaria town, most of those killed or wounded were children. At least 25 people have been killed in the violence, initial estimates by officials said.
Kaduna state has previously seen attacks by Boko Haram. Last Sunday the group attacked two church services, sparking violence which killed seven people. Hundreds have died in its previous attacks on churches.
Boko Haram says it wants Islamist sharia law in place across Nigeria.
A country of 150 million people, Nigeria's population is equally divided between Christians and Muslims.