Twenty-five people have been killed and 49 wounded in a bombing at a chapel next to Egypt's main Coptic Christian cathedral during Sunday mass, according to Egyptian state television.
It is the second deadly attack to hit Cairo in two days.
Egypt's official Mena news agency said an assailant lobbed a bomb into a chapel close to the outer wall of St Mark's Cathedral, seat of Egypt's Orthodox Christian church and home to the office of its spiritual leader, Pope Tawadros II, who is currently visiting Greece.
Egyptian state TV and the Health Ministry gave the casualty toll. Earlier the number of dead was thought to be 22.
Witnesses said the explosion may have been caused by an explosive device planted inside the chapel. Conflicting accounts are common in the immediate aftermath of attacks.
The blast took place as a Sunday mass being held in the chapel was about to end and coincided with a national holiday in Egypt marking the birth of Islam's Prophet Muhammad. Most of the victims are thought to be women and children.
State television aired calls by several Cairo hospitals treating the wounded for blood donations and President Abdel-Fattah el-Sissi declared a three-day state of mourning.
"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.
There was no immediate claim of responsibility for Sunday's attack, which bore the hallmarks of Islamic militants fighting the government of President Abdel-Fattah el-Sisi.
As defence minister, he led the July 2013 ousting of Mohamed Morsi, an elected Islamist president who hails from the Muslim Brotherhood.
An angry crowd of several hundred people gathered outside the cathedral, chanting anti-government slogans and calling for the sacking of the interior minister, who is in charge of security.
Scuffles broke out with the police when the protesters tried to push through their barricades, but there were no immediate reports of arrests. Police in full riot gear later arrived at the scene.
Egypt has seen a wave of attacks by Islamic militants since the military overthrow of former president Morsi, a freely elected leader, in 2013. Many of Morsi's supporters blamed Christians for supporting the overthrow, and scores of churches and other Christian-owned properties in southern Egypt were ransacked that year.
On Friday, six policemen were killed in a bomb attack in Cairo claimed by a group suspected by authorities of links to the outlawed Muslim Brotherhood.