Victory would hand Sisi a six-year term in which immediate priorities would be taming near-record inflation, managing a chronic foreign currency shortage and preventing spillover from the conflict between Israel and Gaza's Hamas rulers.
Voting, which runs from 9 a.m. until 9 p.m. (0700-1900 GMT), is spread over three days, with results due to be announced on December 18.
As voting began on Sunday morning, small crowds gathered at polling stations in Cairo, where pictures of Sisi have proliferated in the weeks leading up to the election. Riot police were deployed at entrances to Tahrir Square in the center of the capital.
Critics see the election as a sham after a decade-long crackdown on dissent. The government's media body has called it a step towards political pluralism.
Three candidates qualified to stand against Sisi in the election, none of them high-profile figures. The most prominent potential challenger halted his run in October, saying officials and thugs had targeted his supporters - accusations dismissed by the national election authority.
Authorities and commentators on tightly controlled local media have been urging Egyptians to turn out to vote, though some people said they were unaware when the election was taking place in the days before the poll. Others said voting would make little difference.
"I was aware there are elections happening, but I had no idea when. I only knew that because of the massive Sisi campaigns around the streets," said Aya Mohamed, a 35-year-old marketing executive.
"I feel indifferent about the elections because there will be no real change," she said.
As army chief, Sisi led the 2013 ousting of Egypt's first democratically elected president, Mohamed Mursi of the Muslim Brotherhood, before being elected to the presidency the following year with 97% of the vote.
Since then he has overseen a crackdown that has swept up liberal and leftist activists as well as Islamists. Rights groups say tens of thousands have been jailed.
Sisi and his backers say the crackdown was needed to stabilize Egypt and counter Islamist extremism. He has presented himself as a bulwark of stability as conflict has erupted on Egypt's borders in Libya, and earlier this year in Sudan and Gaza.
Sisi was re-elected in 2018, again with 97% of the vote.
Soaring prices
Economic pressures, however, have become the dominant issue for Egypt's fast-growing population of 104 million, with some people complaining that the government has prioritized costly mega-projects while the state takes on more debt and citizens struggle to contend with soaring prices.
"Enough of projects and infrastructure, we want the prices to go down, we want for the impoverished to eat and people to have a living," said Imad Atef, a vegetable seller in Cairo.
The election campaign has been low-key, with Sisi following a typical program of opening an arms trade fair, inspecting roads and sitting in on examinations for candidates to join military and police academies in the week leading up to the vote.
Some analysts say the election, originally expected in early 2024, was brought forward so economic changes — including a devaluation of an already weakened currency — could be implemented after the vote.
The International Monetary Fund on Thursday said it was in talks with Egypt to agree additional financing under an existing, $3 billion loan program that had stalled because of delays to sales of state assets and a promised shift towards a more flexible exchange rate.
"All indicators suggest that we're going to move quite quickly after the election in terms of proceeding with the IMF reform," said Hany Genena, chief economist at Cairo Financial Holding, an investment bank.
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