(CNN) -- Egyptian Muslims attended Christmas services Friday in a show of solidarity with Coptic Christians days after a bombing killed 23 congregants in the country's north.
Coptic Christians celebrate Christmas Day according to the Julian calendar, and observed it on Friday this year.
Security was tight around churches after the blast on New Year's Day outside the Church of the Two Saints in Alexandria.
Congregants were forced to empty their pockets before the start of the service as part of security measures set up after the bombing.
"Police plan a large-scale security operation for tonight to protect Egypt's Coptic Christians and their churches." Col. Alla Mahmoud of the interior ministry said Thursday.
Protests have broken out in Christian areas of Egypt every night since the car bombing outside the Church of the Two Saints in Alexandria.
Hundreds of officers -- outfitted in helmets with visors, body armor and carrying shields and night sticks -- lined the streets, leaving a path for demonstrators to pass.
Egyptian authorities have released a sketch of a man they think is responsible for the attack.
"The man in the picture is unknown and authorities are trying to confirm his identity," Mahmoud said.
The interior ministry used forensic technology to recreate the face of the suspected suicide bomber.
About 9 percent of Egypt's 80 million residents are Coptic Christians.
Coptic Christians base their theology on the teachings of the Apostle Mark, who introduced Christianity to Egypt, according to St. Takla Church in Alexandria, the capital of Coptic Christianity.
The religion is known for its rift with other Christians in the fifth century over the definition of the divinity of Jesus Christ.
Journalist Ian Lee contributed to this report from Cairo