The attacks left at least 43 dead and dozens more injured, amid grim scenes of hollowed-out churches, with body parts and blood scattered among the debris.
Outraged Egyptians posted messages of solidarity with members of the embattled religious minority on social media, using a hashtag that translates to "your terrorism brings us together."
Despite tensions between the groups, the country's Muslim community has frequently shown support for Christians following acts of violence. Images on social media showed Muslims gathering inside mosques Sunday to donate blood for victims.
It was reminiscent of an interfaith gesture following the 2011 Egyptian revolt in Tahrir Square. Christians joined hands in a circle around praying Muslims to protect them from regime forces and loyalists trying to break up the protest.
After a deadly Alexandria church bombing in 2011, Egyptian Muslims attended Coptic Christmas services
in a show of solidarity.
'Blood and body parts everywhere'
St. George's Church in Tanta, a small city located between Cairo and Alexandria, had been the target of a bomb threat in March. That didn't deter an estimated crowd of 2,000 congregants from attending Palm Sunday mass, the start of Holy Week before Easter.
The first bomb went off around 9:00 a.m. Church camera footage showed that the choir was in the middle of performing before the feed abruptly cut off.
Peter Kamel was about to leave home for mass when he learned about the explosion and rushed over to look for friends.
"The bombing was so loud that even people who live far away could hear it," Kamel told CNN. "Everything is destroyed inside the church."
The bomb appeared to have exploded near the altar, striking choir members and priests, Kamel said.
On Facebook, Kamel posted scenes of the devastation inside the church:
A pair of sneakers laying among the debris.
Blood splashed on marble pillars, spattered across paintings and woven stalks of green palms.
The bodies of victims, many of them burned, among splintered pews.
Mina Abdel Malak said he was outside St. George's during the explosion and rushed inside to look for his cousin.
"It was horrible; blood and body parts everywhere," he said. "People on the other side of the street felt the explosion shaking their cars."
He rushed to the hospital and saw the name of his cousin, a teacher with two young children, on a list of the deceased.
The previous bomb threat should have put authorities on notice, he said.
"This shouldn't have happened. This is our feast and there was supposed to be strict security measures," he said. "For someone to get this amount of explosives inside, then security wasn't doing its job."
'Thank God it is a Sunday'
Fadi Sami heard about the Tanta bombing during mass at St. Mark's Coptic Orthodox Cathedral in Alexandria. The church had drawn an especially big crowd because Pope of Alexandria Tawadros II was leading prayers.
The Pope did not mention the bombing but Sami said a feeling of uneasiness hung over the congregation. He said he left after the sermon, and 20 minutes later, a suicide bomber blew himself up outside the church's gate.
"It's difficult to process this idea, that if I had left 20 minutes later, I would have stopped to exist in this world," Sami, 26, told CNN.
The blast tore through nearby storefronts, he said. The area was enveloped in smoke as people rushed over to find bodies and body parts scattered among debris, he said.
"I saw a man put together what was left of his son in a bag," he said.
The death toll could well have been worse if it had been a weekday, Egyptian blogger Maged Butter said. Usually, this commercial part of downtown Alexandria is crowded with shoppers.
"Thank God it is a Sunday, and many shops are closed."
He recorded video showing crowds filling the streets as emergency vehicles tried to pass.
Blood stains could be seen meters away from the site of the explosion. Women milled about in search of loves ones; some yelled at police for not protecting them, Butter said.
"Every now and then, I see a person crying -- I think they are Christian -- and they keep saying: Have you seen my family? Have you seen my family?"
A violent act 'against all of us'
After the attacks Egyptians directed their anger at authorities.
Video posted early Sunday on Facebook shows an angry crowd surrounding and beating Maj. Gen. Hussam Ad-Din Khalifa, director of security in Gharbiya Province where Tanta is located, when he tried inspecting the damage at St. George church. Shortly afterward, President Abdel Fatah el-Sisi relieved Khalifa of his duties.
As protesters were gathering outside St. Mark's on Sunday night, volunteers were searching for remains in a secured area nearby.
Meanwhile, Sisi gave a televised speech in which he declared a state of emergency and called for unity.
"What's happening now is against all of us" -- not just Copts, he said. "The main aim is to destroy the unity of our country, Egypt."
"We will defeat terrorist groups, the killers and will continue fighting and building at the same time," he said.