It was the deadliest suspected attack on the country’s beleaguered Christian minority in recent memory.
The explosion ripped through the church around 10:00 am (0800 GMT), Egyptian security officials said, shattering its windows.
State television cited a security official as saying a bomb made of TNT appeared to have been the cause of the explosion.
Ambulances lined up outside the church in the capital’s Abbasiya district to evacuate the dead and wounded.
A crowd gathered outside the church chanting: “Tell the sheikh, tell the priest, Egyptians’ blood is not cheap”.
The church is close to the St Mark’s Cathedral, the seat of Coptic Pope Tawadros II.
Copts, who make up about 10 percent of Egypt’s population of 90 million, have faced persecution and discrimination that spiked during the 30-year rule of Hosni Mubarak, who was toppled by a popular uprising in 2011.
Dozens have been killed in recent years in sectarian attacks and clashes throughout Egypt.
In 2011, a suicide bomber killed 21 worshippers outside a church in the coastal city of Alexandria.
Islamist extremists have regularly incited violence against Copts, especially since the military overthrow of Islamist president Mohamed Morsi in 2013.
They accused the Christians of supporting his ouster, which came after millions of protesters took to the streets demanding his resignation.
On August 14, 2013, mobs attacks dozens of churches across the country after police killed hundreds of pro-Morsi protesters in Cairo. – AFP