They are accused of "attacks against places of worship, resisting authorities, blocking traffic, gathering on the street and damaging public and private property, including by arson," the official said on Sunday.
"Authorities have decided to keep the 103 for more than the usual four-day preventive detention in light of the exceptional nature of the incidents," he added, asking not to be identified.
About 5000 Muslim rioters clashed with police outside the St. Girgis (George) Coptic Orthodox church on Friday, and three were killed and scores wounded.
The violence followed a week of protests over a stage play
deemed offensive to Muslims and performed two years earlier
in St. Girgis Church, which was one of seven churches attacked.
The clashes were among the worst of communal violence Egypt has seen in years, underscoring the fragile religious balance of the Arab world's most populous country.
While passing by unnoticed at the time, the play was recorded by someone and distributed on DVDs recently, angering a large section of Alexandria's predominantly Muslim community.
Riot police clashed with protesters
on Friday (Aljazeera)
Police on Saturday deployed in force around the church.
Church authorities say the Copts account for 10% of Egypt's 72 million-strong population, but the government says they make up 6%.