A series of Christmas Day church bombings rocked Nigeria on Sunday in what appeared to be a coordinated assault by a radical Islamist sect with suspected training links to Al Qaeda, raising the sect’s violent antigovernment struggle to a new and more dangerous level that the Nigerian authorities seem powerless to contain. At least 25 people were killed.

The sect, known as Boko Haram, until now mostly targeted the police, government and military in its insurgency effort, but the bombings on Sunday represented a new, religion-tinged front, a tactic that threatens to exploit the already frayed relations between Nigeria’s nearly evenly split populations of Christians and Muslims.

The worst bombing was at the St. Theresa Catholic Church in Madala, a suburb of the capital, Abuja, where an explosion ripped through a crowd of worshipers as they left morning Mass. The bomb tore through the church, said Bassey Udo, a Nigerian journalist in Madala, and left a deep crater. A government spokesman, Reuben Abati, said at least 25 people were killed in that blast and that many were wounded in a chaos of fire and rubble, suggesting the toll would rise.

At least two other churches in other towns were attacked Saturday evening and Sunday, as well as a police headquarters in the north, an area where dozens have been killed in a bloody conflict in recent days between the security forces and the sect. Officials said the attack on the police headquarters was a suicide bombing and that at least three officers were killed.