A car exploded in front of a Coptic Christian church as worshippers emerged from a New Year's mass in the Egyptian city of Alexandria early yesterday, killing at least 21 people.

After the blast, enraged Christians at the church clashed with police and stormed a nearby mosque, prompting fights and volleys of stone- throwing with Muslims, police and witnesses said – a sign of the sectarian anger that has been arising with greater frequency in Egypt.

Nearly 1,000 Christians were attending the mass at the Saints Church, said its priest, Fr Mena Adel. The service had just ended, and worshippers were leaving the building when the blast went off about a half-hour after midnight.

"I was inside the church and heard a huge explosion," Adel said. "People's bodies were in flames."

The blast came from a car parked outside the church, but police said they were still investigating whether the car had been rigged with explosives or if a bomb had been placed under it.

Alexandria governor Adel Labib blamed al-Qaeda, pointing to recent threats by the terror group's branch in Iraq to attack Christians. "The al- Qaeda organisation threatened to attack churches inside Egypt," he told state TV.

A photographer said protesters stormed into the mosque, throwing books inside out on to the street. The protest sparked clashes with Muslims, as both sides began throwing stones and bottles at each other in the streets.

The attack comes at a time of rising sectarian tension in Egypt and the broader region.