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The number of casualties could be as high as 25 killed and 60 injured, Reuters reports, citing state TV.
Egyptian officials have confirmed that the incident was a bomb explosion, according to AP. The explosive device was pre-planted in the prayer hall, according to Al-Manar broadcaster.
"There was a huge explosion in the hall. Fire and smoke filled the room and the injuries were extremely severe. I saw the intestines of those injured and legs severed entirely from their bodies," Reuters quoted Vivian Fareeg as saying.
Graphic images and footage presumably from inside the church have emerged, showing people gathered around apparently lifeless bodies.
"There was blood all over the floor and body parts scattered," an eyewitness told Reuters.
A second blast occurred near a police training area in the same town, according to local Daily News Egypt.
The deadly attack occurred on Palm Sunday, which commemorates the entry of Jesus Christ into Jerusalem. No group immediately claimed responsibility for the blast.
The attack on the Coptic church comes weeks before the visit of Pope Francis to Egypt, scheduled for April 28-29.
The town of Tanta was the target of a terrorist attack just a few days ago, when 16 people were wounded in a police training center bombing on April 1. A militant group called Liwa al-Thawra, believed to be linked to the outlawed Muslim Brotherhood, claimed responsibility for that assault.
Egypt has faced a surge in terrorist attacks since July 2013, after the country's Islamist president, Mohamed Morsi, was ousted in a military coup. The Ansar Beit al-Maqdis group, which has been affiliated with IS and known as "Wilayat Sinai" since late 2014, has claimed responsibility for the majority of the attacks, according to TASS. The biggest attack claimed by the group was the bringing down of Russian Metrojet Flight 9268 in October 2015. The group has repeatedly targeted Egyptian military and police, as well as the Christian minority, both during full scale terrorist attacks and through individual violence.
One of the deadliest attacks on the Christian minority in Egypt occurred last December, when a local Islamic State (IS, Formerly ISIS/ISIL) affiliate killed dozens of people in a suicide bombing in the largest Cairo Coptic Cathedral. The Christian minority represents 10 percent of the country's 90 million people.
Comment: It is quite clearly no coincidence that Egypt got two major terror attacks the morning after its government condemned US airstrikes against the Syrian airbase.
'ISIS' is no religious army - it's a Western mercenary terrorist group.