Initial reports suggested that the two attacks were conducted by suicide bombers.
The death toll rose to at least 49 Monday, state media reported.
The Serbian prime minister and president-elect reacted in this way in the wake of two terrorist attacks on Christian churches in Egypt that killed 44 people.
A defiant President Abdel Fattah al-Sisi reacted by declaring a three-month state of emergency.
He also announced the formation of a supreme council for fighting terrorism and extremism in Egypt as per a new law to give it sufficient authorities to do the job.
El-Sisi also accused unnamed countries of fueling instability in the country, adding "Egyptians have foiled plots and efforts by countries and fascist, terrorist organizations that tried to control Egypt".
"We are seeing simultaneous attacks, based on strong information, targeting big churches across the country". The attacks were claimed by the Islamic State group, and both bombings were said to have been timed to impact the greatest number of churchgoers.
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Andrew Boyd, a spokesperson for the Christian organisation Release International which supports believers under pressure for their faith, told Premier the Egyptian Church is growing, despite persecution. Isis claims that they have not finished with their attacks yet, and have threatened to do more in a statement they issued. On Sunday, Francis expressed his "deep condolences to my brother, Pope Tawadros II, he Coptic church and all of the dear Egyptian nation".
Hours after the attack, Kerols Paheg and other young Coptic Christians were already digging graves in the basement of the devastated St. George Church in the northern Nile Delta city, where the first of the bombs exploded, killing 27 and wounding around 80. An explosive device had been planted under a seat in the main prayer hall, it said.
Video from inside the church broadcast by CBC TV showed people gathered around what appeared to be lifeless, bloody bodies covered with papers. Its marble pillars were covered with blood, he said.
A policeman was also killed in the church blast in Alexandria. Maher said he didn't recall any unusual movements, but other worshippers told him that they saw a man rushing inside the church before the explosion. "If there were, this wouldn't have happened", she said.
He said women were crying and looking for their loved ones and were yelling at police for "not protecting" them. "The message sent out to Christians is that you are vulnerable wherever and whenever".
Fadi Sami heard about the Tanta bombing as he sat in the Alexandria cathedral on Sunday. She said that she was only able to work according to temporary permissions by the people, or sneak a few shots while no one paid attention. He left as the pope finished the sermon. At least three police officers were killed preventing the suicide bomber from entering the cathedral, Ahram Online reported.