(BBC) - A bomb explosion in the Coptic Christian cathedral complex in the Egyptian capital Cairo has killed at least 25 people, officials say.
Dozens of others were injured in the blast in a chapel adjoining St Mark's cathedral during a Sunday service.
Many of the victims are thought to be women and children.
Egypt's Christian minority has often been targeted by Islamist militants. Christians gathered to show anger at the attack, the worst in years.
Egypt has seen a wave of attacks by militants since 2013 when the military overthrew President Mohammed Morsi, an elected leader who hailed from the Muslim Brotherhood, and launched a crackdown against Islamists.
Some of Mr Morsi's supporters blamed Christians for supporting the overthrow.
The explosion happened at about 10:00 local time (08:00 GMT). Video footage carried by regional media showed the interior of the church littered with broken and scattered furniture, along with blood and clothing on the floor.
"I found bodies, many of them women, lying on the pews. It was a horrible scene," said cathedral worker Attiya Mahrous.
"There were children. What have they done to deserve this? I wish I had died with them instead of seeing these scenes,'' another witness told the Associated Press news agency.
President Abdul Fattah al-Sisi has declared a three-day period of national mourning.
In a statement, he condemned the attack, calling for the perpetrators to be hunted down and punished.
"Vicious terrorism is being waged against the country's Copts and Muslims. Egypt will emerge stronger and more united from this situation," he said.
Coptic Christians make up about 10% of Egypt's population.
St Mark's Cathedral is the headquarters of the Coptic Orthodox church, and the home of its leader, Pope Tawadros II.