ALEXANDRIA, Egypt (AFP) – Coptic Christians marked a sombre Christmas on Friday, after a deadly New Year's Day church bombing in Egypt sparked riots that injured dozens of policemen and protesters.

Egypt has been under tight security since the attack in the northern city of Alexandria killed 21 people, and the measures were stepped up for Christmas Eve services held for Copts on Thursday.

Under the Coptic calendar, Christmas Day falls on January 7.

Security officials said at least 70,000 officers and conscripts had been deployed across the country to secure churches as Copts, who account for 10 percent of Egypt's 80 million population, attended Christmas Eve mass.

Drivers are banned from parking in front of churches, which were being tightly monitored by explosives detection teams and policemen, a police official said.

Police said one primitive explosive device -- a tin can filled with fire crackers, nails and bolts, but without a detonator -- had been found in a church in the southern city of Minya.

Hundreds of worshippers gathered on Thursday at the Saints Church in Alexandria, the site of Saturday's bombing. They were guarded by dozens of police and anti-riot vehicles.

In Alexandria, 27-year-old Maureen, dressed in black, said: "To survive, we Copts must confront our fear and pain. We have to be stronger than the terrorists. That's why I am coming to mass."

Maher, 50, arrived for the mass with his wife and two daughters. "Our sorrow is great, but we feel stronger because of the support of our Muslim compatriots," he said.

Others converged on Saint Mark's Cathedral in Cairo, where the head of the Coptic Church, Pope Shenuda III, conducted the service, attended by several government members and Egyptian President Hosni Mubarak's two sons Alaa and Gamal.

In Moqattam, an impoverished Cairo district with a large Coptic population, residents said the threat of further attacks would not deter them from going to church.

"With Al-Qaeda's threats, we anticipate further attacks but we are not afraid. God protects us," said Adel al-Wazir.

Police have released a sketch of the suspected Alexandria suicide bomber's face, reconstructed from the remains of a severed head found on the roof of the church.

No one has yet claimed responsibility for the attack, which came after threats to Egypt's Copts from an Al-Qaeda-linked group in Iraq that had said it was behind a deadly October assault on a Syriac Catholic church in Baghdad.

The group, the Islamic State of Iraq, said it would attack Copts if their church failed to release two women it claimed were being held against their will after converting to Islam.

Several weeks before the attack, a website linked to Al-Qaeda published a list of Coptic churches it said should be targeted in Europe and Egypt, including the one bombed on January 1.

Several other countries, including Canada, France, Germany and the Netherlands stepped up security around Coptic Christian churches in response to the threat.

At the Saint Mark's Coptic Church in the Canadian city of Montreal Robert Mishriky, a businessman and church official, said a private security firm was hired for Christmas Eve mass for the first time.

"Tonight, we used a private security firm which sent three officers, one to watch for cars and two inside the church to watch bags and monitor people. We have never done that before," Mishriky said.

Some 250,000 Copts live in Canada, which has 14 Coptic churches.

A security official in Jordan, home to an estimated 3,000 Copts, told AFP police in the capital Amman had tightened security for Christmas services at two Coptic churches after the Alexandria attack.

The bombing sparked days of protests and riots around Egypt that injured dozens of policemen and protesters.

President Mubarak has vowed to find those responsible for the Alexandria church bombing, which he blamed on "foreign hands."