Egypt's president says a suicide bomber carried out the bombing at Cairo's Coptic Orthodox cathedral complex on Sunday that killed at least 25 people.
Egyptian President Abdel Fattah el-Sissi said authorities arrested three men and a woman in connection with the blast.
The president was speaking during a state funeral for the victims.
There has been no claim of responsibility for the attack.
A VOA reporter at the scene said the attack occurred during the Sunday morning service 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 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.
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.
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 for the Associated Press.