Sunday's bombings, claimed by the Islamic State group, are the latest escalation by the extremist group - which recently vowed to step up its attacks against Egypt's embattled Christian minority.
The emergency law grants the president special powers, which include: censoring and confiscating publications; monitoring and intercepting all forms of communication; closing any facility; and imposing restrictions on the freedom of movement for any citizen. Abdullah had detonated his explosives at the church's entrance.
So-called Islamic State (IS) said it was behind both explosions.
Eyewitnesses said a police officer detected the bomber before he blew himself up. At least 28 people died and 77 were wounded. Roshdy was due to get married in a month.
After the bombings, Sisi ordered military deployments to guard "vital and important infrastructure", his office said.
The United States issued a travel warning on Wednesday, advising its nationals in Egypt to stay away from places of worship for the next two weeks and avoid crowds.
"I heard the blast and came running". El-Haggar was conducting inspections for people entering the church. "She was protecting Christians who were also praying in the Church". We also pray for our Coptic Orthodox sisters and brothers who continue to be resilient in the face of ongoing and escalating attacks, and who resist the urge to react vengefully or reciprocally.
Palm Sunday falls on the Sunday before the Easter.
With Pope Francis' upcoming visit to the country and IS suffering setbacks in Syria and Iraq, it is likely that the suicide bombings were intended as prelude to a wave of terrorist attacks targeting Egypt's Christians.
Highlighting the many messages of sympathy he had received from around the world since yesterday's attacks, he said: "Prayer is the most important thing we can ask for at this time".
It added that security forces killed seven suspected militants in a shootout on Monday as they met to plan attacks on minority Christians.
"So sad to hear of the terrorist attack in Egypt". President Mohamed Morsi imposed it in January, and following his ouster, his successor Adly Mansour imposed it in August.
Steffen Seibert told reporters that it was important to preserve the peaceful coexistence of Muslims and Coptic Christians in Egypt.