Egypt is holding seven people for questioning over the New Year's Day bombing of a Coptic church in the northern city of Alexandria and has released 10 others, a security source said on Sunday.

Another source said questioning was continuing related to the attack, which killed 21 people outside the church during a midnight service. He said a number of suspects had been detained and most were held briefly before being freed.

The suspected suicide bomber wounded 97 people in the blast, which prompted hundreds of Christians in Muslim-majority Egypt to protest against a failure of the authorities to protect them.

Pope Benedict, head of the Roman Catholic church, condemned the bombing as a "vile gesture," the latest in a series of attacks on Christians in the Middle East and Africa.

Egyptian officials said there were indications that "foreign elements" were behind the blast and said the attack seemed to have been the work of a suicide bomber.

Extra police officers were posted outside several churches in Cairo and Alexandria on Sunday, preventing cars from parking next to the buildings, witnesses said.