The underdog in Egypt's presidential election, Hamdeen Sabahy, has withdrawn his delegates from polling stations nationwide to protest an election commission's decision to extend the voting for a third day.
However, Sabahy - the only challenger to the leading contender, former general Abdel-Fattah al-Sissi - stopped short of quitting the race.
The commission's decision, announced hours before polls were due to close on Tuesday, came amid reports of a low turnout in the two scheduled days of voting.
"My responsibility and duty oblige me to highlight to you the necessity of completing what we have begun out of belief in our right to establish a course of democracy," Sabahy, a leftist politician, said in a statement to his backers on Wednesday.
His campaign charged that the extension came after pressure "to intervene in the numbers and percentages voting in the elections".
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The election commission said it was acting in response to growing numbers of people coming to the polls on Tuesday in what were to have been the final hours of voting. It also cited daytime temperatures that hit 40C in parts of Egypt as discouraging voters from turning out or waiting until closing times neared.
Polling stations opened on Wednesday for the final day of the voting, which al-Sissi is expected to win easily.
His supporters see him as able to end the turmoil that has hit Egypt over the past three years.
Turnout was weak in the first hour of the balloting on Wednesday, according to local media.
State television and pro-military broadcasters were airing patriotic songs and calling for voters to cast their ballots.
Egypt's interim authorities are anxious for a high turnout to win legitimacy for al-Sissi at home and abroad.
About 37 per cent of eligible voters had shown up in the first two days, according to the commission.
The election is Egypt's first since July when the army, which was then led by al-Sissi, deposed Islamist president Mohammed Morsi, whose Muslim Brotherhood has called for a boycott of this election.
Nearly 52 per cent of the registered voters cast their ballots in the 2012 polls, which Morsi won.
An estimated 53.9 million eligible voters are being asked to turn out at 14,350 polling stations spread across Egypt's 27 provinces.
Polls are due to close at 9pm local time (0400 AEST Thursday) with final results expected to be announced on June 5.
