Sisi, who has been in power since 2014, appears set to run unopposed after all other presidential hopefuls were either jailed or announced they would not take part in the election on March 26-28.
Nominations for candidates remain open until Monday.
The undersigned "condemn all security and administrative practices that the current regime took to prevent any fair competition against it in the upcoming elections," they said in the statement.
The document was signed by a barred presidential hopeful's top aides -- Hisham Geneina, a former anti-corruption chief, and Hazem Hosni, a political science professor at Cairo University.
They had been part of the team campaigning for General Sami Anan, a former armed forces chief of staff.
Anan was accused of illegally announcing, on January 20, his intention to run for president before getting the military's approval.
Sunday's call for voters to boycott the election was also signed by 2012 presidential candidate Mohamed Anwar Sadat, a dissident and nephew of the former president of the same name.
On January 15, Sadat said he would not throw his hat into the ring this time because the climate was not conducive to free and fair elections.
The boycott call was also signed by moderate Islamist and 2012 presidential candidate Abdel Moneim Abol Fotouh, a former senior leader of the Muslim Brotherhood.
The fifth signatory was Essam Heggy, a NASA space scientist who worked as an adviser to former interim president Adly Mansour.
Mansour had served as acting president after Sisi led the 2013 military ouster of former Islamist president Mohamed Morsi prior to being elected president the next year.
The five signatories cited "the vicious attack against Justice Hisham Geneina", who was stabbed and seriously wounded by assailants on Saturday.
Lawyer Ali Taha said three men stabbed Geneina in the face and beat him near his home in Cairo, breaking one of his legs. He was admitted to hospital for treatment.
Geneina was sacked by Sisi as head of the Central Auditing Authority in 2016 after he was accused of exaggerating the cost of corruption.
They said "obstacles to the elections had started early with the spreading of a climate of fear over security and media and government bias."
The five also complained of a "tight timetable which did not give competitors a real chance to present themselves."
The National Election Authority announced on January 8 that the deadline for applications would be January 29.
They requested the elections be halted, and for the authority's work to cease, accusing it of covering up bias.
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