The attacks came two months after the Islamic State released a video showing its militants pledging to kill Coptic Christians across Egypt.
Fadi Sami, another witness, was inside the Alexandria cathedral when he heard about the church bombing in Tanta, when the Coptic pope was leading the prayers for Palm Sunday.
After the carnage, there was chaos. A few hours later, a security camera captured the second suicide bombing at a church in Alexandria.
In February, Islamic State group militants threatened more attacks against Copts, who make up about 10 percent of Egypt's population.
Elsadi said the world won't be able to put an end to terrorism as long as there are certain governments practicing or legalizing or endorsing terrorism, as the case in Syria, or stand inactive in the face of terrorist groups growth in order to sell more weapons.
Some victims of Sunday's attack directed their anger against the government.
CBC TV showed footage from inside the church in Tanta, where a large number of people gathered around bodies covered with papers.
Leaders of the church have taken precautionary measures and vamped up security at the church in case the unthinkable happens.
April 2013: Two people were killed outside St Mark's cathedral in Cairo when people mourning the death of four Coptic Christians killed in religious violence clashed with local residents. They say they also mourn for the Muslims who were also killed at the hands of ISIS.
As the congregation prepares for Easter, they will now be remembering both Jesus Christ and the lives lost in this tragedy.
President Trump, who hosted Sisi last week in his first official visit to the U.S., expressed support for a leader he has said he plans to work more closely with on fighting Islamist militants, who Sisi identifies as an existential threat.
"So even though it's been hard, we just laud our brothers and sisters in this land at the dignity and the patience and the grace they have exhibited during their times of suffering", he said.
Egyptian officials denounced the violence as an attempt to sow divisions, and Francis sent his "deep condolences" to Tawadros.
Before the beginning of the Mass, Francis and a group of cardinals, holding elaborately braided palm fronds, walked through the crowd at Saint Peter's Square.
Copts believe that their Church dates back to around 50 AD, when the Apostle Mark is said to have visited Egypt. Mark is regarded as the first Pope of Alexandria - the head of their church.
"IS and its supporters online have been methodically introducing more radical sectarian concepts to Egyptian jihadists since the December bombing", said Mokhtar Awad, a research fellow with George Washington University's Program on Extremism.
Hudson County and New Jersey politicians expressed their support for the Coptic Christian community after two churches were bombed in Egypt over the weekend.