Egypt’s interior minister charged Sunday that a Palestinian extremist group with links to Al Qaeda was behind the Dec. 31 bombing outside a church in Alexandria that killed 21 people and set off days of sectarian rioting around the nation.

In a nationally televised speech, the minister, Habib el-Adly, said the authorities had “conclusive evidence” linking the attack on Egyptian Christians to the Army of Islam, a militant group based in the Gaza Strip.

Mr. Adly said that investigators had arrested several Egyptian men connected to the attack and that they had provided details about how they were recruited by the Gaza group.

“These despicable terrorist acts will not get the better of the will of the nation and the nobility of Egypt, where the principles of moderation and the values of tolerance and acceptance of others and renouncing violence and terrorism has taken root in the consciousness of its people through the centuries,” Mr. Adly said in a speech that marked Police Day, which will be observed Tuesday.

Late Sunday, the Interior Ministry identified one of those arrested as Ahmed Lotfy Ibrahim, 26, from Alexandria. A ministry statement said he had visited Gaza in 2008, where he met members of the extremist group who influenced him to attack churches in Egypt. The authorities said Mr. Ibrahim maintained contact with the group until the time of the attack.

A spokesman for the group interviewed by telephone denied any involvement in the terrorist strike in Alexandria.

The Army of Islam is believed to have participated in the cross-border raid in 2006 to capture the Israeli soldier Gilad Shalit, who remains a prisoner.

A spokesman for Hamas, the militant group controlling Gaza, also denied the charge and said that it had asked mediators to contact the Egyptian authorities to offer full cooperation with the investigation.

“We affirm that there is no Al Qaeda in Gaza and that all Palestinian factions and the resistance direct their guns to the Zionist enemy only,” said a government spokesman, Taher Al-Nounou.

The bomb attack occurred as worshipers left a New Year’s Eve Mass in Alexandria. No one has claimed responsibility for the bombing, but immediately afterward, government officials said they detected “foreign hands” behind the explosion.

That charge was immediately criticized by many Christians and Muslims alike, who said it demonstrated that the government was continuing to deny what had become evident: that serious tensions existed between the Muslim majority and the Christian minority, which is about 10 percent of the population.

The government struggled for three days after the attack to quell rock-throwing protests by Christians and their supporters in cities around the country.

Since then, the authorities have moved with uncommon speed to resolve some issues that have angered Christians. A man who had fired on a crowd of worshipers leaving a Christmas Mass in Nag Hammadi a year ago, killing seven people, was sentenced to death. The government also quickly put on trial a police officer who opened fire on a train, killing a Christian man and wounding several others, after the bombing in Alexandria.

“I think it’s more to relieve the pressure that is on them and to show that they are doing something,” said Mona Makram-Ebeid, a former member of Parliament from a prominent Christian family in Cairo. “It’s not convincing, it’s not enough, but we should encourage any step they take in this direction.”

The effort to defuse sectarian tensions came as Egypt faced the prospects of more civil unrest as early as this week. Government opponents have organized a demonstration for Tuesday in Cairo and while such protests are generally not well attended, usually under pressure from security services, more than 75,000 individuals noted on a Facebook page that they planned to participate.