An explosion at Cairo's Coptic Orthodox cathedral complex Sunday has killed at least 25 people and wounded nearly 50 others.
MENA, Egypt's official news agency, reports an assailant threw a bomb into a chapel at St. Mark's, in Cairo's Abbassia district.
There has been no claim of responsibility for the attack.
A VOA reporter at the scene said the attack happened during the 10 a.m. service Sunday in an area with mostly women.
Video posted to social media showed pools of blood beneath debris inside the church of St. Peter and St. Paul, known in Arabic as El-Botroseya.
Egypt's President Abdel-Fattah el-Sissi declared a three-day state of mourning after the blast.
"The pain felt by Egyptians now will not go to waste, but will result in an uncompromising decisiveness to hunt down and bring to trial whoever helped through inciting, facilitating, participating or executing this heinous crime,'' a presidential statement quoted the Egyptian leader as saying.
WATCH: Cathedral attack reaction
More than 500 Coptic protesters gathered at the scene of the explosion, deriding the government and calling for the president to step down. The demonstration prompted small skirmishes with police until a priest called for calm. Dozens of Muslim supporters joined the Christian protesters.
The attack came two days after a bomb elsewhere in Cairo killed six policemen, an assault claimed by a group authorities link to the outlawed Muslim Brotherhood.
St. Marks is the seat of Egypt's Orthodox Christian church and home to the office of its spiritual leader, Pope Tawadros II, who is currently visiting Greece.
Human rights groups and the media have regularly documented cases of anti-Christian violence in the country, including a 2011 attack on New Year's Day at a Coptic church in Alexandria that killed 20 people.
Copts make up about 10 percent of Egypt's population of 93 million.
Hamada Elrasam contributed information from Cairo to this report, which includes additional information from the Associated Press.