* Milestone in political road map after 2011 popular revolution
* Referendum follows toppling of elected Islamist president
* Army chief Sisi seen running for president after vote
* Military re-ascendant, stunting democratic transition (Adds start of vote, quote from polling station)
By Michael Georgy
CAIRO, Jan 14 (Reuters) - Egyptians began voting on Tuesday in a constitutional referendum, the first ballot since the military overthrew Islamist president Mohamed Mursi and an event likely to spawn a presidential bid by army chief General Abdel Fattah al-Sisi.
Few doubt that Egyptians, who staged mass protests against Mursi's rule before his ouster, will turn out in big numbers and vote "yes" in the two-day referendum, a milestone on the army-backed government's political road map.
Sisi ousted Mursi, Egypt's first freely elected head of state, in July. Sisi's Islamist foes see him as the mastermind of a coup that kindled the worst internal strife in Egypt's modern history and brought back what critics call a police state.
But many Egyptians are weary of the political upheaval that has gripped Egypt and shattered its economy since they rose up to topple autocrat Hosni Mubarak in 2011, and they view Sisi as a decisive figure who can reinstate stability.
Analysts say the referendum is also turning out to be a vote on the popularity of a man whose image features on posters across Cairo.
If he runs for president, Sisi is widely expected to win.
In the Nile Delta town of Tanta, about 40 women lined up at a polling station at a girl's primary school. About a dozen policemen and soldiers kept watch.
"The acceptance of this constitution will show that we had a glorious revolution on 30 June that was the will of the people implemented by the army," said Negla Hassan, a 30-year-old English teacher.
Egyptians who support the military takeover refer to it as a revolution, arguing that it represented the popular will because it followed mass protests against Mursi's rule.
"We have removed the Brotherhood and we will live with stability and security with our police and our army. With Sisi at the head of our country, Egypt will be the mother of the world again," added Hassan.
Egypt's political transition may well keep stumbling. Al Qaeda-linked Islamist militants, which security officials say have ties to the Brotherhood, have stepped up attacks against security forces since Mursi's ouster.
The Brotherhood has repeatedly said it is a peaceful movement and that it hoped street protests would bring down the government and its allies in the military.
But the severe security clampdown - hundreds of Islamists have been killed and thousands arrested - has taken the steam out of demonstrations. Mursi and many other top Brotherhood leaders have been arrested and are on trial.
An explosion was heard in Cairo just before voting was due to begin, security sources said on Tuesday. The blast occurred near a court complex in the district of Imbaba. There were no early reports of casualties.
State and private television channels demonise the Brotherhood as "terrorists" while frequently showing Sisi inspecting military sites, giving speeches and meeting with members of the public.
The referendum will mark the third time Egyptians have voted on constitutional arrangements since the historic uprising against Mubarak, a former air force chief, in January 2011, and overall the sixth time they have gone to the polls since his downfall.
The constitution will replace one signed into law by Mursi a little more than a year ago after it was approved in a referendum. The new text strips out disputed Islamist language while strengthening state institutions that defied Mursi: the military, the police and the judiciary.
Egypt's Western allies were hoping that a more competitive political field would emerge, three years after the Arab Spring tide of democratic change swept through the country.
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But little leverage has been applied to back their calls for inclusive politics in the biggest Arab nation. Suspicions have already emerged about the fairness of the referendum.
Egypt is deeply influential in the Arab world and whatever political scenario unfolds there could impact the region.
The political turmoil has wrecked the economy. But Gulf Arab states, suspicious of the Muslim Brotherhood, jumped to Egypt's rescue after Mursi's overthrow, providing billions of dollars in aid.
Approval of the rewritten constitution appears a foregone conclusion. Mursi's now-outlawed Muslim Brotherhood is calling for a boycott rather than a "no" vote, while many Egyptians who backed his removal are expected to vote "yes" in support of the army-backed order that has replaced Islamist rule.
The International Commission of Jurists (ICJ), a Geneva-based group that works to uphold the rule of law, described the draft constitution as highly flawed.
"The referendum campaign has taken place within a context of fear, intimidation and repression, calling into question the fairness of the entire process," it said in a statement.
The referendum anchors a transition plan the government unveiled in July with the stated aim of restoring democracy, although it simultaneously launched a fierce crackdown on the Brotherhood, Egypt's best organised party until last year.
Driven underground and declared a terrorist group on Dec. 25, the Brotherhood has said it will shun the road map. A presidential vote is expected as early as April, once the referendum is approved, with a parliamentary election later.
Egyptian government officials have hailed the revised constitution as a clear sign of democratic progress. Human rights groups are highly sceptical.
Human Rights Watch expressed concern at reports that seven activists from the Strong Egypt party face criminal charges for hanging posters calling for a 'no' vote in the referendum.
"Egyptian citizens should be free to vote for or against the new constitution, not fear arrest for simply campaigning for a 'no' vote," said Joe Stork, Middle East and North Africa deputy director at Human Rights Watch. "Protecting the right to vote requires safeguarding the right to free expression." (Additional reporting by Tom Perry and Maggie Fick, editing by Mark Heinrich and Hugh Lawson)