Hundreds of Christian demonstrators paraded through the streets of Alexandria after a car bomb killed 21 worshippers celebrating the New Year.
Bearing a blood-stained banner of Jesus rescued from the shattered church, protesters clashed with Egyptian riot police who had moved in to break up the gathering.
Christians and Muslims pelted each other with rocks and cars were also torched in the disturbances.
'We are not going to remain silent,' chanted the protesters. Three demonstrators were arrested and beaten up by police, according to witnesses.
Two statues of Jesus and the Virgin Mary were toppled and benches scattered by the impact of the blast.
A wooden cross hanging on the church gate was covered with a white sheet stained with victims' blood and bits of human flesh remained stuck on the gate.
Young Christian men prevented cleaners from removing the flesh. 'Leave them. This is pure blood,' one of the men shouted.
There were also several demonstrations in Cairo where Aida Seif al-Dawla, a veteran activist, called for the Interior Minister to be held accountable for the failure to protect the church.
Violence: Other protesters carried crosses as they came up against a line of Egyptian riot police
Sally Moore, another Christian protester, said Muslim and Coptic protesters are planning to form a 'human shield' outside major churches in Cairo on Coptic Christmas Eve on January 6 in a show of solidarity.
'The security is protecting the regime, not the people, not the churches,' she said.
Around 1,000 people were attending Mass at the Saints church when the bomb went off just after midnight, causing total carnage.
As well as the 21 dead, at least 70 were injured. Witnesses described it as 'a scene from Baghdad.'
Kameel Sadeeq, from the Coptic council in Alexandria, said: 'People went in to church to pray to God but ended up as scattered limbs.
'This massacre has Al Qaeda written all over, the same pattern Al Qaeda has adopted in other countries.'
The blast also damaged a mosque near the church. Eight Muslims were among the wounded.
Egypt's president Hosni Mubarak has urged Muslims and Christians to stand together against terrorism, claiming the attack bore the marks of 'foreign hands'.
The attack has been condemned as 'barbaric and heinous' by Barack Obama. In Rome, Pope Benedict XVI said the attack 'offends God and all of humanity.'
Christians in Muslim-majority Egypt make up about 10 percent of the nation's 79 million people.
Egypt, due to hold a presidential election in September, has stepped up security around churches after a threat from an Al Qaeda-linked group in November.
The Islamic State of Iraq claimed that the church was holding women who had converted to Islam.
No group has claimed responsibility for the attack but security officials said they were looking at the possibility that homegrown Islamic extremists were behind it, and perhaps were inspired by Al Qaeda though not directly under foreign command.
Investigators were also examining lists of air passengers who arrived recently in Egypt from Iraq because Al Qaeda in Iraq threatened Christians in both countries.
They said they are looking for any evidence of a financier or organiser who may have visited Egypt to recruit the bomber and his support team from local militants.
Investigators were also examining two heads found at the site on suspicion that at least one was the bomber's, state news agency MENA reported.
The crime lab investigation found the explosives used were locally made and were filled with nails and ball bearings to maximise the number of casualties.
Egypt's government has long insisted that al-Qaida does not have a significant presence in the country, and it has never been conclusively linked to any attacks here.
Egypt does, however, have a rising movement of Islamic hard-liners who, while they do not advocate violence, adhere to an ideology similar in other ways to Al Qaeda.
There have been fears they could be further radicalized by sectarian tensions. The hard-liners, known as Salafis, have a large and active presence in Alexandria.
The security officials cautioned, however, that the attackers may not necessarily have come from the ranks of the Salafis but more likely came from one of a number of small, fringe groups that are even more radical.
Alexandria, a famed city of antiquity which a century ago was home to a mix of Muslims, Christians, Jews and foreigners, has become a stronghold for Islamic hard-liners in the past decade.
Stabbings at three Alexandria churches in 2006 sparked three days of Muslim-Christian riots that left at least four dead.
The security officials, who spoke on condition of anonymity because the investigation has not yet been completed, also said 25 people have been detained for questioning, but none of them was thought to be linked to the attack.
They said the 25 were mostly owners of cars parked outside the church at the time, storekeepers and Muslim neighbors known to be Islamic fundamentalists.
Suspicion for the attack immediately fell on Al Qaeda after the terror group's branch in Iraq vowed to attack Christians in Iraq and Egypt over the cases of two Egyptian Christian women who sought to convert to Islam.
The women, who were married to priests in the Coptic Orthodox Church, were prohibited from divorcing their husbands and sought to convert as a way out.
The women have since been secluded by the Coptic Church, prompting Islamic hard-liners in Egypt to accuse the Church of imprisoning them and forcing them to renounce Islam. The Church denies the allegation.
Al Qaeda in Iraq cited the Egyptian women in a claim of responsibility for the attack on a Baghdad church in October that killed 68 people.
The group has also threatened Egypt's Coptic Orthodox Christian community over the two cases and the church attacked was an Orthodox Coptic church.
The attack was dramatically different from past violence against Christians, which included shootings but not serious bombings, much less suicide attacks.
Christians, mainly Orthodox Copts, make up about 10 percent of Egypt's mainly Muslim population of nearly 80 million.
The attack only served to heighten tensions that have been growing in recent years between Christians and Muslims.