Egypt is holding seven people for questioning over the New Year's Day bombing of a church in the northern city of Alexandria and has released 10 others, a security source said on Sunday.
Another source said questioning was continuing related to the attack, which killed 21 people outside the church during a midnight service. He said "a number" of suspects had been detained and most were held briefly before being freed.
The suspected suicide bomber wounded 97 people in the blast, which prompted hundreds of Christians in Muslim-majority Egypt to protest against a failure of the authorities to protect them.
Pope Benedict, head of the Roman Catholic church, condemned it as a "vile gesture," the latest in a series of attacks on Christians in the Middle East and Africa.Egyptian officials said there were indications "foreign elements" were behind the blast and said the attack seemed to have been the work of a suicide bomber.
Extra police officers were posted outside several churches in Cairo and Alexandria on Sunday, preventing cars from parking next to the buildings, witnesses said.
An Iraqi group linked to al Qaeda threatened the Church in Egypt with attack in November and a statement on an Islamist website, posted about two weeks before the Alexandria bombing, urged Muslims to attack Coptic churches in Egypt and elsewhere.
One security source said seven people were being detained, and 10 had been released after questioning.
"There are people being held and investigated. This is part of the investigations to reveal the mysterious circumstances of the incident and gather information," said the second source, who declined to give specify how many were being detained.
President Hosni Mubarak, 82, has pledged to track down the culprits and called for national unity, saying the attack was directed at all Egyptians, not just Christians.
Dozens of Christians gathered inside a cathedral compound on Sunday to demand the state and church do more to help them.
One protester, Nader Shenouda, said: "When there was a threat from al Qaeda a month or a month and a half ago, did the government have to wait till the disaster happens before they (the government) protect us?"
Their protest coincided with a meeting between the pope and Sheikh Ahmed El-Tayeb, the head of al Azhar, Egypt's most prestigious seat of Sunni Muslim learning. The sheikh expressed condolences.
Christians make up about 10 percent of Egypt's 79 million people. Tensions often flare between Christians and Muslims over issues such as building churches or close relationships between members of the two faiths.
Analysts said the attack was on a much bigger scale than typical sectarian flare-ups but said laws that make it easier to build a mosque than a church, and similar causes of Christian complaint, meant such an attack would fuel sectarian tension.
"Right now Copts feel Muslims (as a whole) struck at them, rather than seeing it as a terrorist attack by one Muslim, and it is due to this … feeling of discrimination," said Hisham Kassem, a publisher and rights activist.
