Sisi said that the war against the jihadists will be "long and painful", and that the state of emergency will be established after taking all "legal and constitutional steps".
The Islamic State group claimed the bombings at two churches in Tanta and Alexandria cities as worshippers observed the Palm Sunday.
The attack at St. Mark's Cathedral took place just outside of the entrance to the church and occurred during a Palm Sunday service given by the head of the Egyptian Christian community, Coptic Pope Tawadros II, the New York Times reported.
The attacks came a week before Coptic Easter and Pope Francis' scheduled arrival in the country.
Egypt's Health Ministry says a bomb has struck a church north of Cairo, killing at least 13 people and wounding 25 others.
Security cameras showed this suicide bomber outside St. Mark's Cathedral in the coastal city of Alexandria before he killed 11 people.
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A number of world leaders have condemned the attack.
Sisi also ordered troops be immediately deployed to assist police in securing vital facilities, a rare move for the general-turned-president, who as defence chief led the military's 2013 ouster of the Muslim Brotherhood's President Mohamed Mursi. "This terrorist attack is devoid of all the principles of humanity and civilisation", it said in a statement.
According to reports, the suicide bombings killed atleast 45 and injured many.
ISIS, on the other hand, issued a statement via the Telegram messaging platform and said the suicide bombers behind the Palm Sunday attacks were Egyptian nationals.
Outraged Egyptians posted messages of solidarity with members of the embattled religious minority on social media, using a hashtag saying "your terrorism brings us together". "The members of the Security Council reaffirmed that terrorism in all its forms and manifestations constitutes one of the most serious threats to global peace and security". "May the Lord convert the heart of those who sow terror, violence and death and also the heart of those who make weapons and trade in them", he added. Trump spoke by phone Sunday with el-Sissi to express confidence that Egypt will do what it can "to protect Christians and all Egyptians".