Three suicide bomb attacks on churches rocked a northern Nigerian state Sunday, killing at least 12 people and wounding about 80, officials said, prompting protests in a state that has previously been strained by religious tensions.

The first two blasts occurred within minutes of each other and targeted two churches in the city of Zaria, said Kaduna State police chief Mohammed Abubakar Jinjiri. A third blast hit a church in the city of Kaduna about half an hour later, Jinjiri said.

The Zaria attacks killed the bomber and at least one other person and left 51 wounded, said Nigerian Red Cross official Andronicus Adeyemo. The Kaduna attack claimed 10 more lives, he said, and wounded 29 people.

Jinjiri said security at the three churches prevented the suicide bombers from ramming explosive-laden cars into the buildings filled with worshippers.

"If not for security, there would have been [many] more casualties," Jinjiri said.

Churches have been increasingly targeted by violence in Nigeria. The situation has led churches in Nigeria's predominantly Muslim north to boost their security in a nation of more than 160 million people almost equally divided between Muslims and Christians.

Attacks on religious holidays

Last weekend, a suicide car bomber detonated his explosives outside a church in central Nigeria as gunmen attacked another church in the nation's northeast, killing at least six people and wounding dozens of others.

Attacks on Christian holidays have claimed the most lives. An Easter Day blast in Kaduna left at least 38 people dead. A Christmas Day suicide bombing of a Catholic church in Madalla near Nigeria's capital killed at least 44 people.

Police arrested one of the bombers who survived. Jinjiri declined to say who police suspected might be responsible, though a radical Islamist sect known as Boko Haram has claimed similar church attacks in the past.

Boko Haram, whose name means "Western education is sacrilege" in the Hausa language of Nigeria's north, is waging an increasingly bloody fight with security agencies and the public. More than 560 people have been killed in violence blamed on the sect this year alone, according to an Associated Press count.

The Nigerian Red Cross said young people had started protesting in Kaduna, leading the state government to impose a 24-hour curfew in a state deeply divided along religious lines. An Associated Press reporter also saw billows of smoke over a mosque in a predominantly Christian part of the city.

People had mounted illegal roadblocks and were seen harassing motorists. A motorcycle rider in that same neighborhood lay seriously hurt and bleeding by the road side. Motorbike riders there are often presumed to be Muslim and become easy targets during reprisal attacks by Christians.

Kaduna state, which sits on Nigeria's dividing line between its largely Christian south and Muslim north, was at the heart of postelection violence in April 2011. Mobs armed with machetes and poison-tipped arrows took over streets of Kaduna and the state's rural countryside after election officials declared President Goodluck Jonathan the winner. Followers of his main opponent, former military ruler Muhammadu Buhari, a Muslim, quickly alleged the vote had been rigged, though observers largely declared the vote fair.

According to Human Rights Watch, at least 800 were killed in the postelection violence. Of the 800, at least 680 people were killed in Kaduna State alone.