Egypt was on high alert on Monday ahead of the Coptic Christmas holiday following a New Year\'s Day church bombing that killed 21 people, as investigators raced to identify those behind the attack.
Police cancelled leave for top officers and were tightening surveillance of airports and ports to prevent suspects from leaving the country, as new checkpoints were set up across the nation.
Security was also to be beefed up at churches for Christmas which Copts celebrate on January 7, security officials said.
The clampdown comes amid concerns of new protests by Copts following overnight clashes at Cairo\'s St. Mark\'s Cathedral -- headquarters of Coptic leader Pope Shenouda III -- during which 45 policemen were wounded.
The protesters pelted with stones a minister who had come to visit the pope and heckled government officials, while other demonstrators blocked off four main streets in Cairo before police dispersed them.
The violence died down on Monday, but about 30 protesters prevented a construction crew from repairing damage at the site of the blast, witnesses said.
They said the blood of the victims should remain to bear testimony to the attack.
In Cairo, about 50 protesters carrying wooden coffins held a brief demonstration in central Cairo.
Coptic Christmas will fall on Friday -- the weekly Muslim day of prayer and rest -- and Shenouda said he intended to say mass as usual on Christmas Eve.
"Not praying would mean that terrorism has deprived us of celebrating the birth of Christ," the official Al-Ahram newspaper quoted him as saying.
Twenty-one people were killed early on New Year\'s Day and 79 wounded when an apparent suicide bomber detonated his payload as hundreds of worshippers were leaving midnight at Al-Qiddissin (The Saints) church in Alexandria.
A security official said on Sunday that about 20 people were detained for questioning but there was no evidence any of them was directly connected to the attack.
Al-Ahram quoted security officials as saying that the bomb was a sophisticated device packed with TNT and pieces of metal to cause the largest number of casualties possible.
No one claimed responsibility for the attack, which came two months after an Al-Qaeda linked group which claimed responsibility for a deadly Baghdad church hostage-taking threatened Coptic Christians.
The group demanded the release of two women, Camelia Shehata and Wafa Constantine, both priests\' wives, it said the church was holding against their will after they converted to Islam.
An Al-Qaeda-linked website that published that threat had posted in December a list of Egyptian churches it said should be attacked, including the church targeted in the bombing.
It urged "every Muslim who cares about the honour of his sisters to bomb these churches during Christmas celebrations, when they will be most crowded."
Egypt, the most populous Arab country, witnessed a resurgence in attacks by Islamist militants in the last decade after the government battled a spate of attacks in the 1990s that included an attempt abroad to kill the president.
President Hosni Mubarak has vowed to find those responsible for the bombing which he said targeted all Egyptians, regardless of their faith, and blamed "foreign hands."
The bombing has further underscored the vulnerability of the Copts, who make up about 10 percent of the country\'s 80-million population and complain of discrimination.
Last year began with a massacre of six Copts and a Muslim security guard after a Coptic Christmas Eve mass and ended on a tense note after two Coptic protesters died in clashes in a protest over a Cairo church permit.
Some Coptic activists have accused the government of not doing enough to prevent incitement against the minority, especially after Islamists began staging regular demonstrations demanding the release of Shehata.
Shehata, like Constantine in 2004, had escaped her husband last year and reportedly wanted a divorce, something that is very difficult to obtain from the Coptic church.
The church has denied that either of the women converted. Women\'s rights activists say Coptic women have been known to convert, either to Islam or to another Christian denomination, to escape unhappy marriages.
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