Polls opened on Saturday in a landmark referendum on an amended constitution intended to take Egypt through free and fair elections after the ouster of veteran president Hosni Mubarak last month.
Just five weeks after the veteran strongman quit, an estimated 45 million eligible voters have the chance to give their verdict on the transitional military government's plans for a swift return to civilian rule, and a high turnout is expected.
Polls opened at 8 am (0600 GMT) in a referendum in which voters are asked to say "yes" or "no" to a package of constitutional changes intended to guide the Arab world's most populous nation through fresh presidential and parliamentary elections within six months.
The vote has divided Egypt between those who say much deeper constitutional change is needed and others who argue that the amendments will suffice for now.
The Muslim Brotherhood, a well organized Islamist group, has come out in favor of the amendments, setting it at odds with secular groups and prominent reform advocates including Mohamed ElBaradei and Amr Moussa, both candidates for the presidency.
ElBaradei, speaking at a conference in New Delhi, said the referendum on changes to the constitution dealt only with "minutiae".
"It doesn't talk about the imperial power of the president, it doesn't talk about the distortion of the parliament, it doesn't talk about the need to have an independent constituent assembly that represents everybody," he said.
"So we are going to say no tomorrow. Most of the people who triggered the revolution are going to say no. I take a flight tonight to cast my vote to make sure that this will not happen."
The military council to which Mubarak handed power on Feb. 11 is hoping the amendments will pass so it can move along the path it has set towards parliamentary and presidential elections that will allow it to cede power to an elected government.
An appointed panel of experts drew up the proposed amendments in just 10 days, as the military council which took over on Mubarak's resignation strives to hand over the reins of power as quickly as possible and keep the army above the political fray.
But the hasty, improvised nature of the proposed constitutional underpinnings of Egypt's promised new democracy has driven many of the leading groups and figures behind the victorious protest movement to urge a "no" vote.
On the eve of the referendum on Friday, thousands of activists returned to Cairo's iconic Tahrir Square, focal point of the protests against Mubarak, to demonstrate their opposition.
Most of the amendments are by themselves uncontroversial, although critics argue they do not go nearly far enough in overhauling the Mubarak-era charter, which they say needs to be completely rewritten.
The president would serve a maximum of two four-terms and would no longer have the power to refer civilians to the military courts.
The state of emergency which has governed Egyptian life for decades could only be imposed for six months without endorsement in a popular referendum.
Restrictions on who can stand for president would be eased, if not entirely relaxed, and judicial supervision of all elections would be restored to prevent vote-rigging.
The head of the judicial commission overseeing the referendum, Mohammed Atteya, hailed it as among "the first fruits of the revolution" which overthrew Mubarak's regime at the cost of at least 384 lives.
"This is the first time in Egyptian history voters would be participating in a political process that is both credible and transparent," he said.
But the youth groups, which spearheaded the protest movement, and a host of secular political parties and opposition figures, say the timetable being set by the military is too tight for movements stymied by three decades of authoritarian rule to organize at grass-roots level.
The military itself has been studiously non-partisan, urging "yes" and "no" supporters alike to turn out to vote.
"Accepting or rejecting the amendments is the right of each Egyptian. Cast your vote to preserve the gains of the January 25 revolution," it said in a statement on its Facebook page.
There have been no opinion polls in the run-up to the referendum and assessments of its likely outcome have been as divided as views about the proposed changes.
Some analysts predict a majority "yes" vote, at least outside the big cities, given the strong support of the Brotherhood, and the perceived backing of the army, whose popularity is running high after it sided with the protesters against Mubarak.
Others are more skeptical, pointing to the widespread economic discontent in the provinces that has sparked a wave of strikes and walkouts.
Voters need only a national identity card to cast their ballot, even one that has expired, and can do so in any polling station. Indelible ink is to be used to prevent multiple voting.
Some 34,000 troops are to join police in providing security.