* Last-minute extension seen as bid to boost ex-army chief
* Worry that low turnout could undermine his legitimacy
* Opposition rejects extra voting as manipulation
* Sisi vows to fight Islamist militants, rescue economy
By Asma Alsharif and Shadia Nasralla
CAIRO, May 28 (Reuters) - Egyptians were urged to vote on a hastily added third day of a presidential election on Wednesday after lower-than-expected turnout threatened to damage the credibility of the man widely forecast to win, former army chief Abdel Fattah al-Sisi.
After months of adulation by the media encouraged by his supporters in government, the security services and business, many Egyptians were shocked when the election failed to rally the expected mass support predicted by Sisi himself.
For Sisi, locked in a battle with the Muslim Brotherhood after toppling Islamist president Mohamed Mursi last year, the stakes are high.
Poor backing in the election in his deeply polarised country would mean Sisi's legitimacy as head of state of the Arab world's most popular nation would be harmed at home, in the Middle East and in the wider world.
The two-day vote was originally due to conclude on Tuesday at 10 p.m. (1900 GMT) but was extended until Wednesday to allow the "greatest number possible" to vote, state media reported.
Distancing Sisi from the vote extension, seen by commentators as an embarrassing attempt to attract every last vote from a reluctant electorate, his campaign announced that he had objected to the decision.
Unlike the previous election which brought Mursi to power and was contested by a dozen candidates, Sisi faces only one rival now: the leftist Hamdeen Sabahi.
Sabahi's campaign rejected the extension of voting as unjustified, given the lack of enthusiasm shown so far by many Egyptians.
In a statement, it denounced the move as an attempt "to prevent the Egyptians from expressing their opinion through manipulating the turnout rates and the voting percentages".
Some voters doubted whether any further support for Sisi might be found, despite the extension. "I came to see the millions they said were coming to vote. I can't see anybody except two people and the electoral commission," said Hussein Hassanein, a 24-year-old law student.
"I won't vote for either. This is a fake election. We know that Sisi is going to win. Who would you expect me to vote for?"
Lines outside polling stations in various parts of Cairo were short on Tuesday, and in some cases no voters could be seen, even though the military-backed government had launched a determined effort to get out the vote, declaring Tuesday a public holiday.
The justice ministry said Egyptians who did not vote would be fined, and train fares were waived in an effort to boost the numbers. Local media loyal to the government chided the public for not turning out in large enough numbers.
One prominent television commentator said people who did not vote were "traitors, traitors, traitors".
Al-Azhar, a state-run body that is Egypt's highest Islamic authority, said a failure to vote was "to disobey the nation", state TV reported. Pope Tawadros, head of Egypt's Coptic church, also appeared on state TV to urge voters to head to the polls.
Turnout in the 2012 election won by Mursi was 52 percent - a level this vote must exceed for Sisi to enjoy full political legitimacy, said Hassan Nafaa, a professor of political science at Cairo University.
Were it to fall short, then he will have failed "to read the political scene and his miscalculation has to be corrected through reconciliation", he said. Sisi had called for a turnout of 40 million, or 80 percent of the electorate.
Sisi's supporters see him as the man to rescue the Arab world's largest country after three years of upheaval. He became a hero to many for removing Mursi after mass protests.
But the Islamists accuse him of staging a bloody coup, and a broad crackdown on dissent has alienated liberal young people who were behind the revolution that toppled Hosni Mubarak during the 2011 "Arab Spring".
Trying to lower sky-high expectations in the run-up to the election, Sisi stressed the need for austerity and self-sacrifice, a message that cost him some support and drew some ridicule in a nation of 85 million steeped in poverty.
He had announced his priorities as fighting Islamist militants who have taken up arms since Mursi's removal, and reviving an economy badly in need of tourists and investors.
On Sisi's Facebook page, admirers posted hundreds of pictures of themselves wearing Egyptian flags or patriotic T-shirts, with ink on their fingers to show they had voted for him. Others had banners saying "long live Egypt", Sisi's slogan.
He is the sixth military man to run Egypt since the army overthrew the monarch in 1952. (Additional reporting by Maggie Fick, Stephen Kalin and Samia Nakhoul in Cairo, Abdelrahman Youssef in Alexandria and Mahmoud Mourad; Writing by Tom Perry; Editing by Giles Elgood/Mark Heinrich)