Mousa Mostafa Mousa leads the Ghad party, which had endorsed Sisi for a second term and even organized events to help nominate the former military commander as recently as last week.
Mousa said he would mount a full challenge to Sisi, though opposition activists, journalists, and analysts took to Twitter to dismiss him as a dummy candidate, standing only to give the impression of a full democratic contest.
“This is all theater,” said a shopkeeper outside the Ghad party headquarters in downtown Cairo.
Sisi was elected in 2014, a year after leading the army to oust President Mohamed Mursi, an Islamist. It is the third election since protests in 2011 unseated long-time ruler Hosni Mubarak.
Several leading opposition figures have called for a boycott of the election, slated for March 26-28, saying repression had cleared the field of challengers and left Sisi’s top opponent in jail.
Even before campaigning has officially begun, the United Nations, rights groups and opposition figures criticized the run-up as compromised by arrests, intimidation of opponents and a nomination process stacked in favor of the incumbent.
At a news conference Mousa said the late decision to run came after all other candidates withdrew. He dismissed accusations that his candidacy was being used to present a false sense of competition.
“We are entering a fair and honorable competition in order to win,” said Mousa.
The electoral commission has said it will ensure the vote is fair and transparent.
Would-be candidates were required to register by 2 p.m. (1200 GMT) on Monday after clinching at least 20 nominations from parliament or 25,000 pledges from citizens across the country.
Mousa submitted his official paperwork just minutes before the final deadline. He said he had netted more than 47,000 pledges and the backing of 27 lawmakers.
Sisi’s campaign said on Monday it had received nomination pledges from 915,000 citizens. More than 500 of parliament’s 595 lawmakers had already pledged support for Sisi.
“We have a respectable program that we are offering to the Egyptian people and we are presenting ourselves just like any other candidate would,” the deputy head of the Ghad party, Mahmoud Mousa, said.
Earlier this week Hisham Genena, a former anti-corruption watchdog chief who had been working to elect former military chief of staff Sami Anan, was attacked and badly wounded outside his home.
Anan’s campaign was considered the strongest challenge to Sisi to date. It was abruptly halted after he was arrested last week and accused of running for office without military permission.
As late as Sunday it appeared likely that Sisi would have no electoral opponents, even as a group of prominent figures spanning the political spectrum announced the first major call to boycott the vote.
Members of Anan’s campaign, former Islamist presidential candidate Abdel Moneim Abol Foutoh, and Mohamed Anwar Sadat, who halted his own presidential bid this month, called for the election to be suspended and said state policies were “removing any opportunity for the peaceful transfer of power”.
“The Egyptian state does not want this to be described as a referendum internationally, which it would be if there is no challenger whatsoever to Sisi on the ballot,” said H.A. Hellyer, senior non-resident fellow at the Atlantic Council.
“Even if that challenger is someone like Mousa Mostafa Mousa, who is on record as backing Sisi for a second term – the optics seem to be assessed as less problematic if he does run,” Hellyer said.
“It’s going to be an intense competition,” wrote one Twitter user with a screenshot of Mousa’s Facebook page that included a photo of Sisi’s face and the slogan “we support you as president of Egypt” written beneath it.