At least 21 people were killed and 43 injured when a car bomb went off outside a church in the northern city of Alexandria, according to official estimates on Saturday, dpa reported.
Witnesses said that a car that was parked outside the church exploded around 20 minutes after midnight, targeting Coptic Christians who were attending the New Year's Eve mass.
A security source told the German Press Agency dpa that there were around 100 kilograms of explosives attached to the car.
A group of angry Christians threw stones at a mosque nearby, leading to clashes with the police.
Hosny Mubarak has called on Egyptians " Copts and Muslims to stand united against terrorism and those who threatens security, stability and unity of the country."
The Islamic State of Iraq, a group affiliated with al-Qaeda, has recently threatened Christians throughout the Middle East, saying that they are legitimate targets.
The group demanded that pressure be put on Egypt's Coptic Church to "release" two Egyptian women, which they said had converted to Islam and were now being forcibly held by the Coptic church.