A bombing at Egypt's main Coptic Christian cathedral has killed 25 people and injured another 49, according to Egyptian state television, making it one of the deadliest attacks carried out against the religious minority in recent memory.

Key points: Six children among 25 killed in St Mark's Cathedral blast

Six children among 25 killed in St Mark's Cathedral blast Witnesses describe "horrible scene" and flesh on the floor

Witnesses describe "horrible scene" and flesh on the floor No immediate claim of responsibility

There was no immediate claim of responsibility for Sunday's attack, which killed at least six children, security sources said.

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.

However, 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.

An Associated Press reporter who arrived at the scene shortly after the blast saw blood-stained pews and shards of glass scattered across the chapel's floor.

Men and women wailed and cried outside the chapel.

Members of the special police forces stand guard to secure the area. ( Reuters: Mohamed Abd El Ghany )

"I found bodies, many of them women, lying on the pews. It was a horrible scene," said cathedral worker Attiya Mahrous, who rushed to the chapel after he heard the blast.

His clothes and hands were stained with blood and his hair matted with dust.

"I saw a headless woman being carried away," Mariam Shenouda said as she pounded her chest in grief.

"Everyone was in a state of shock. We were scooping up people's flesh off the floor.

"There were children. What have they done to deserve this? I wish I had died with them instead of seeing these scenes."

Witnesses said the explosion may have been caused by an explosive device planted inside the chapel. ( Reuters: Amr Abdallah Daish )

Orthodox Copts, who make up about 10 per cent of Egypt's 90 million people, are the Middle East's biggest Christian community. They have long complained of discrimination under successive Egyptian leaders.

Islamists are waging an insurgency led by Islamic State's branch in North Sinai, where hundreds of soldiers and police have been killed. The insurgents have also launched deadly attacks Cairo and other cities.

On Friday, two roadside bombs, one in Cairo and one north of the capital, killed six policemen and wounded six others.

AP/Reuters