A bomb blast killed at least 25 worshippers during Sunday mass inside a Cairo church near the seat of the Coptic pope who heads Egypt's Christian minority, state media said.
The explosion ripped through the church at around 10:00 am, Egyptian security officials said.
It was the deadliest suspected attack on the country's beleaguered Christian minority in recent memory.
The blast in the church adjacent to Saint Mark's Coptic Cathedral in the Abbasiya neighbourhood shattered its glass windows and left a scene of carnage in the main prayer hall.
State television quoted a security official saying a bomb made of TNT appeared to have been behind the explosion.
"Tell the sheikh, tell the priest, Egyptians' blood is not cheap," a crowd outside the church chanted, as ambulances lined up to evacuate the casualties.
The church "is deeply loved by many Coptic faithful in Cairo and it has a regular parish presence," said Bishop Angaelos, the General Bishop for the Coptic Church in Britain.
He said services had been held in the church on Sunday morning, while the adjacent St Mark's Cathedral was being renovated.
"It's an easier target because its entrance is outside the precincts" of the cathedral.
There was no claim of responsibility for any attack, but jihadists in Sinai have targeted Christians before, as well as Muslims they accuse of working with the government.
In April 2013, two people died in clashes outside St Mark's Cathedral, seat of Pope Tawadros II.
Copts, who make up about 10% of Egypt's population of 90mn, have faced persecution and discrimination dating back to 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.
With security forces as the main target, militants have repeatedly attacked policemen and soldiers since the army overthrew Islamist president Mohamed Mursi in 2013 and unleashed a bloody crackdown on his followers.
Most of the attacks are conducted in the Sinai Peninsula by a branch of the Islamic State group, although militants have also targeted security forces and government officials in Cairo.
On Friday, six people died in a bombing next to a police checkpoint in the Talibiya area of Cairo shortly before Muslim weekly prayers when the streets are mostly empty.
Turning their sights on the Copts, Islamists in August 2013 attacked churches and homes of Copts in retaliation for the deadly dispersal by security forces of protest camps of Mursi supporters in two Cairo squares.
Enraged by the crackdown in which hundreds died, mobs of alleged Islamists lashed out at Copts in the Upper Egypt province of Minya, accusing them of backing the military.
More than 40 churches were attacked nationwide after the crackdown, with most attacks in Minya and Assiut, Human Rights Watch said at the time.
The Copts were also often targeted during Mubarak's rule.
In 2011, a suicide bomber killed 21 worshippers outside a church in the coastal city of Alexandria.
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