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.
(Two Copts fight with a man (R), who onlookers say was a Muslim, prior to clashes between Coptic youths and riot police, January 1.)
The suspected suicide bomber wounded 97 people in the blast. There was no immediate claim of responsibility for the bombing.
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.
One security source said seven people were being detained, and 10 had been released after questioning.
(A woman grieves for a relative who died in Saturday's bomb attack at the Coptic Orthodox church in Alexandria.)
"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.
(An Egyptian Christian tries to get inside a mosque using a knife, beside a Coptic Orthodox church in Alexandria, 230 km (140 miles) north of Cairo, January 1.)
Sheikh Ahmed El-Tayeb, the head of al Azhar, Egypt's most prestigious seat of Muslim learning and the pope met. The sheikh expressed condolences.
The attack was the worst violence against Egypt's Christian minority in a decade.
It sparked clashes between riot police and Christians who say the government hasn't done enough to protect them.
The Copts are the biggest Christian community in the Middle East and account for up to 10 per cent of Egypt's 80 million population.