After prayers, the bodies were carried out of the heavily-guarded ceremony in white coffins bearing wreaths of white flowers.
Relatives of the victims cried and held each other for support.
All but one of those killed were members of the same family, according to a list of the victims' names released by the church, which said among the dead were a boy and a girl, age 15 and 12 respectively.
Coptic Orthodox Church spokesman Boulos Halim said the death toll was likely to rise.
ISIS militants have for years been fighting security forces in the Sinai Peninsula and along Egypt's porous desert border with Libya. It said the attack was revenge for the imprisonment by Egyptian authorities of "our chaste sisters" but did not elaborate. The previous attack in May past year left almost 30 people dead.
Although accounts of the events are numerous, the official spokesperson of the church stated that gunmen ambushed a microbus and bus transporting Coptic Christians worshipers near the Monastery of Saint Samuel the Confessor in Upper Egypt's Minya on Friday.
"The UAE stands in solidarity with Egypt with the government and people of sisterly Egypt in confronting extremism and terrorism, which seek to undermine Egypt's stability and its national unity, " said H.H. Sheikh Abdullah bin Zayed Al Nahyan, Minister of Foreign Affairs and International Cooperation, in a statement today.
Later on Friday, an IS-linked website reported that militants in Egypt with links to the group had claimed responsibility for the attack.
Security forces are now searching for the unknown gunmen who carried out the terror attack on the bus, the sources said. The cathedral is the seat of Coptic Pope Tawadros II.
A wounded Egyptian woman speaks with a Coptic priest following the attack.
On Saturday, hundreds gathered at the Prince Tadros church in Minya, a city some 260km south of the capital, Cairo, to bury six members of the same family who were shot dead in the attack.
Local church officials in Minya province where the attack took place on Friday put the death toll at 10, but the higher figure could not be confirmed.
According to the Archbishop in Minya, the attack took place in nearly the same place as one by Islamist militants which killed 28 Christians in May 2017.
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