At least 21 people were killed in bomb blast outside a church in the Egyptian city of Alexandria early on New Year's Day and the Interior Ministry said a foreign-backed suicide bomber may have been responsible.
Dozens of people were wounded by the blast, which scattered body parts, destroyed cars and smashed windows. The attack prompted Christians to protest on the streets, and some Christians and Muslims hurled stones at each other.
Egypt has stepped up security around churches, banning cars from parking outside them, since an al Qaeda-linked group in Iraq issued a threat against the Church in Egypt in November.
Egypt's leaders were quick to call for unity, wary of any upsurge in sectarian strife or other tension as the country approaches a presidential election due in September amid some uncertainty about whether President Hosni Mubarak, 82, will run.
Mubarak promised in a televised address that terrorists would not destabilise Egypt or divide Christians and Muslims. He said the attack "carries evidence of the involvement of foreign fingers" and vowed to pursue the perpetrators.
A statement on an Islamist website posted about two weeks before the blast called for attacks on Egypt's churches, listing among them the one hit. No group was named in the statement.
US President Barack Obama described the bombing as a "barbaric and heinous act" and said the United States, a major ally, was ready to help Cairo in responding to it.
The Muslim Brotherhood, seen as Egypt's biggest opposition group and which decades ago renounced violence as means to power in Egypt, condemned the attack.
"There are people who want this country to be unstable, and all fingers point to outside hands being behind this incident," senior group member Mohamed el-Katatni said.
The circumstances of the attack, compared with other incidents abroad, "clearly indicates that foreign elements undertook planning and execution," the Interior Ministry said.
"It is likely that the device which exploded was carried by a suicide bomber who died among others," it said in a statement. State media had earlier blamed a car bomb.
The embassy of the United States, a close ally of Egypt, expressed condolences to vctims of the "terrible event". Other Western and regional states also condemned the bombing.
An Iraqi deputy interior minister, Hussein Kamal, urged Arab states to cooperate in the fight against terrorism and to help stop Arab militants training in Iraq and then returning home.
Health Ministry spokesman Abdel Rahman Shahin said 21 people had been confirmed killed so far and 97 were wounded, the official Middle East News Agency reported.
The church said 20 people were confirmed killed and remains had been found indicating 4-5 others died in the blast, which struck as worshippers marking the New Year left the church.
"We condemn this unfortunate incident that threatens our nation, its security and safety of its citizens. What happened is a dangerous escalation of sectarian events that target the Copts," said a statement from the Alexandria Council of Priests.
Christians make up about 10 percent of Muslim-majority Egypt's 79 million people. Tensions often flare between the two communities over issues such as building churches or close relationships between members of the two faiths.
Analysts said this attack was on a much bigger scale and appeared far more organised than the kind of violence that usually erupts when communal frustrations boil over.
After protests overnight, more than 100 Christians protested again on Saturday near the Coptic Orthodox church that was hit. "We sacrifice our souls and blood for the cross," they chanted. Police used teargas to disperse protesters.
Egypt's Christians have been threatened by Al Qaeda-linked Islamic State of Iraq, which attacked a church in Baghdad two months ago in what it called a response to the mistreatment of Muslim converts by Egyptian Copts.
A statement posted on an Islamist website called on Muslims to "bomb churches during the Christmas holiday when churches are crowded". It was not clear who was behind the statement that listed churches in Egypt and elsewhere, including Alexandria's Church of the Two Saints that was targeted.
The Orthodox Coptic Christmas is on Jan. 7.
Pope Benedict, the head of the Roman Catholic Church, denounced violence against Christians in his New Year address.
Analysts said they did not expect a return to the kind of Islamic militant insurgency crushed by Egypt's government in the 1990s. Nevertheless, the attack could add to sectarian tension and points to influence of foreign Islamist groups.
"The first and most likely possibility is that a sleeper cell of al Qaeda group carried out this operation and this would mean that al Qaeda has penetrated the Islamic political movement in Egypt," said analyst Nabil Abdel-Fattah.
Alexandria governor Adel Labib "accused al Qaeda of planning the bombing", state television reported.
Officials are swift to play down sectarian differences and have been keen to emphasise national harmony before the September presidential poll.
Mubarak, 82 and in power since 1981, is expected to run if he is able to. Gallbladder surgery in March revived questions about his health, but he has returned to a full schedule.
Sectarian tension is fuelled in part by Christian grievances such as laws making it easier to build mosques than churches.
In November, hundreds of Christians clashed with police, and with some Muslims who joined in, in Cairo in a protest against a decision to halt construction of a church. Officials said the Christians had no licence to build. Two Christians died, dozens were hurt and more than 150 detained.
Last January, a drive-by shooting of six Christians and a Muslim policeman at a church in southern Egypt sparked protests.