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Coptic Christians have celebrated Christmas Eve amid tight security, following a bomb attack on a church in Egypt in which 23 people died.

Armed police protected churches as Copts, who mark Christmas according to the Julian calendar, gathered in large numbers, many wearing black.

Some Muslims held vigils outside Coptic churches in a gesture of solidarity.

The services passed off peacefully, despite some radical Islamist websites urging more attacks.

Heightened security measures were expected to remain in place on Friday for Coptic Christmas Day.

The bombing of the al-Qiddissin, or Saints', Church in Alexandria on New Year's Day was the worst act of sectarian violence in Egypt in a decade.

It triggered days of protests and riots by Christians blaming the government for encouraging discrimination and not doing enough to protect them.

In response, the Egyptian authorities stepped up security around many churches, with explosives experts on hand.

Armoured vehicles have been stationed in key areas.

Earlier on Thursday, one device containing nails and fireworks was found in a church staircase in the southern city of Minya but it had no detonator.

Police in Cairo, Alexandria and other places checked the identities of those entering churches.

A street away from the Saints Church in Alexandria, two dozen Muslims held a solidarity gathering holding banners saying "No to terrorism, yes to citizenship" and "Long live the cross and the crescent."

One Muslim woman, Hanan Mahfouz, told the Associated Press at a Mass in a Cairo suburb: "Coming here seemed like the least I could do."

However, radical Islamist websites have been circulating lists of Coptic churches in Egypt and Europe with instructions on how to attack them.

"Blow up the churches while they are celebrating Christmas or any other time when the churches are packed," says a line from a video attributed to al-Qaeda, entitled Jihadi Encyclopaedia for the Destruction of the Cross, that has been widely circulated on the internet.

Pope Benedict XVI used his weekly address in St Peter's Square in the Vatican on Thursday to send "heartfelt greetings and best wishes to our brothers and sisters of the Eastern Churches who will celebrate Christmas tomorrow".

In the wake of the Egypt attack, he said: "May the goodness of God... strengthen the faith, hope and charity of everyone and give comfort to the communities that are being tested."

Some European governments have also announced security measures at churches.

It was during Coptic Christmas Eve celebrations in southern Egypt in 2009 that six Christians and a Muslim security guard were killed in a drive-by shooting.

The country's Coptic Christian minority makes up between 7% and 14% of Egypt's 80 million people.