CAIRO - Christians clashed with Egyptian police in the northern port city of Alexandria on Saturday, enraged after an apparent suicide bomber killed at least 21 Christian worshipers and wounded at least 97 others earlier in the day, Egypt's Health Ministry said. The attack marked the worst violence against Egypt's Christian minority in more than a decade.

As hundreds of Coptic Christian worshipers were leaving a New Year's Mass at Saints Church shortly after midnight, a bomber stepped out of a car and detonated himself in the crowd, according to the Interior Ministry. Egyptian officials and Muslim leaders quickly condemned the attack.

A graphic video of the bombing shared on YouTube showed terrified people running out of the church to investigate as others screamed in terror. At least one car was in flames. Blood-stained sheets covered the dead.

Some Egyptian officials blamed foreign forces, with Alexandria Gov. Adel Labib and Ali Eddin Hilal, the spokesman for President Hosni Mubarak's ruling party, singling out al-Qaeda. The Sunni terrorist network's affiliate al-Qaeda in Iraq has asserted responsibility for a string of recent attacks on Christians in that country and threatened Egypt's Coptic Orthodox community.

If al-Qaeda's involvement were proved, it would raise new security concerns in Egypt, an Arab nation where the government has long insisted that the extremist group does not have a foothold. It would also raise questions about the scope of al-Qaeda in Iraq's influence outside Iraq.

In the group's deadliest attack on Christians last year, militants stormed a Baghdad church in October, killing dozens. The group vowed to mount more attacks, citing the case of two Egyptian Christian women who had reportedly converted to Islam to obtain divorces, a practice the Coptic Church prohibits. It accused the church of imprisoning and isolating the women and forcing them to disavow Islam, which the church denies.

In a statement issued Saturday, the Coptic Church did not attribute the attack to foreign forces, calling it instead an "escalation of sectarian violence against Copts," according to the Egyptian daily al-Masry al-Youm.

Sectarian violence is not new to Egypt, but the latest attack was much deadlier than those in the recent past. In April 2006, 78-year-old Nushi Atta Girgis, a Coptic Christian, was stabbed to death in one of a rash of attacks on churches in Alexandria. In January 2010, six deacons were killed in a drive-by shooting as they walked out of a church in Naga Hammadi, in southern Egypt, after a Coptic Christmas Mass.

Mubarak condemned Saturday's attack in a statement on national television and urged "Egypt's sons, Copts and Muslims to close ranks against terrorism." Egyptian officials were quick to describe the bombing as an attack against "all Egyptians" in an apparent bid to prevent sectarian fallout.

On Saturday, Christians in Alexandria chanted that they would protect the cross with their blood as police used tear gas and rubber bullets to break up the crowd, al-Masry al-Youm reported.

At the St. Mina Monastery, about 18 miles west of Alexandria, at least 10,000 people walked in a procession following eight ambulances that carried the coffins of the victims, local Egyptian media reported.

Meanwhile, in a New Year's message from the Vatican, Pope Benedict XVI condemned the apparently growing violence against Christians in the Middle East and urged the faithful to remain strong.

Bayoumi is a special correspondent. Correspondent Leila Fadel contributed to this report from Portland, Maine.