CAIRO- Egypt is screening people who arrived recently from countries where al-Qaeda is known to recruit after early findings suggested the militant network was behind a New Year's church bombing, security sources said.

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In Europe, authorities said they were pursuing threats against Coptic churches there, after militants said they would attack the Egyptian Christian denomination both in Egypt and among its diaspora communities.

A suspected suicide bomber killed 21 people and wounded 97 outside a Coptic church in the Nile delta city of Alexandria during a New Year's midnight service. The authorities have been holding seven people for questioning.

The bombing prompted protests in parts of Cairo and Alexandria. On Saturday and Sunday, hundreds of Christians in Muslim-majority Egypt took to the streets to protest against what they say is the failure of authorities to protect them.

Hundreds of Christians protested in Cairo's Shubra district, which has a large Christian population and many churches, and a similar protest erupted on Cairo's ring road highway.

The highway protesters initially set car tyres on fire and used them to block the road, and were seen later throwing stones at homes, cars and people.

Security forces dispersed the crowds in both places, but they kept re-assembling and continuing their protest.

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

"The security forces have confirmed that finger of suspicion indicates that the culprit was a suicide bomber linked to al Qaeda," a security source, who asked not to be identified, said.

Another source said police had stepped up security at Egypt's ports and airports to prevent anyone who might have been involved from fleeing as the investigation continues.

"Security is preparing a list of those who have arrived in Egypt recently from countries where al Qaeda is known to recruit operatives," the second security source said.

Threats

Copts, the native Christians of Egypt, are the biggest Christian community in the Middle East and have diaspora communities in the United States and Europe. They worship in a nearly extinct language descended from ancient Egyptian.

The attack came two months after al Qaeda-linked insurgents in Iraq attacked a Baghdad church and threatened to strike Coptic churches in Egypt, accusing the Egyptian Christian denomination of mistreating female converts to Islam.

Two weeks ago, a statement on Islamist websites urged Muslims to attack Coptic churches in Egypt and among Egyptian Christian communities in Germany, France, Britain and elsewhere around Christmas, which Orthodox Copts celebrate on January 7.

A statement after the blast on another Islamist website read: "This is the first drop of heavy rain, hand over our prisoners and turn to Islam." No group was named.

The German government said it had warned Coptic Christians living in Germany about the risk of possible attacks hours before the New Year's blast in Alexandria.

"I can confirm that on New Year's Eve the interior ministry contacted the bishop of the Coptic Christians in Germany, Anba Damian, to tell him about this risk," German Interior Ministry spokesman Stephan Paris told reporters.

A French court said it had opened an investigation after a priest in the greater Paris region complained of online threats against Coptic Christians.

French police will try to work out whether the threats, posted on social networking websites, amounted to a conspiracy to commit acts of terrorism, the court said. It did not identify the websites or give details of the threats.