The turnout was the second lowest of the relentless series of five nationwide elections that Egyptians have been called to in the 22 months since Mubarak's fall. The highest was nearly 60 percent in the election of parliament's lower chamber. The lowest was an embarrassing 8 percent for the vote for the upper chamber, a largely toothless body, which the public cares little about.
Besides the low turnout, preliminary results show that the "yes" vote carried the first round only by a slim margin of 56 percent hardly the resounding endorsement the Islamists were looking for to silence the increasingly vocal and united opposition that called on supporters to vote "no."
For weeks, legal experts in Egypt have said if turnout is low and the final majority in favor of the constitution is lower than 70 percent, it would raise damaging questions about how representative the document is of the nation.
But a spokesman for Islamist President Mohammed Morsi dismissed any talk of thresholds.
"The new constitution will be adopted and go into effect if the result of the referendum is 50 plus 1 percent of votes," said Yasser Ali. "This is what is in the election law and it has not been changed."
The charter has raised furious emotion in Egypt the past three weeks, sparking violence between the Islamist camp and their opponents that left at least 10 dead and 1,000 wounded. The 63-page, 236-clause document would increase the implementation of Islamic law in Egypt, raising critics' fears of sharp limitations on many civil rights.
Opponents were outraged that Morsi's Islamist allies pushed the draft to approval in hasty, dead-of-night session of the Constituent Assembly that wrote the document, despite a boycott by the assembly's liberal and Christian members. A significant sector of the public saw it as the latest of a series of moves by the Muslim Brotherhood, from which Morsi hails, to monopolize power.
Hundreds of thousands of people held street protests across the country demanding Morsi call off the referendum. When he refused, the opposition turned to trying to rally the public to a "no" vote.
Morsi's Islamist supporters brought out their own mass rallies, accusing the opposition of tying to thwart the democratic process. They claim they have a mandate to bring greater Islamic rule because of their victories in last winter's parliamentary elections.
Proponents have campaigned furiously for a "yes" vote some pushing the idea it would bring stability after months of turmoil, and many others taking a religious tack, equating support for the charter with support for Islam.
Bassem Sabry, a prominent blogger who does extensive vote analysis, said Saturday's low turnout suggested a "growing frustration with the entire political process."
"For all the polarization and amassing of supporters over the past few weeks, most of the country appears unaligned politically in any staunch manner and remains composed primarily of swing and/or disinterested voters," he wrote in a blog posted Monday.
The 17 provinces voting in the second round are largely conservative, raising expectations turnout could be somewhat higher though likely not enough to make up for the low first round numbers. If the second round mirrors the first it would mean around 16 million of Egypt's 51 million voters participated a small fraction of the country's population is 85 million.
The low participation could reflect voter fatigue. Since Mubarak's fall, Egypt has held a referendum on initial, transitional constitutional amendments and votes for the two houses of parliament and for the president, which each had run-off rounds and now the current referendum.
But pollster Magued Osman, head of the independent Egyptian Center for Public Opinion Research, said that shouldn't have been the case in Cairo and Alexandria the country's two biggest cities since they "have been key in the debate over the constitution."
Of Cairo's 6 million registered voters, only 2.1 million voted, down from 3.6 in the presidential runoff in June, he said.
He and other experts said one factor could have been skepticism about the integrity of the vote after most judges, in a protest against Morsi, refused to oversee the referendum, as is customary in Egypt.
Along with the shortage of monitoring judges, the short time to prepare for the vote Morsi announced the referendum's date on Dec. 1 led to chaos in most polling centers.
Many of the judges who agreed to oversee the vote were loyal to the president and his Brotherhood. Activists have accused them of looking the other way while violations marred the vote.
Some accused pro-Brotherhood authorities of trying to suppress the "no" vote. In some opposition districts of Cairo and Alexandria, voters had to wait in line for up to 10 hours, with some leaving in frustration before casting their ballots.
"It is not so far-fetched to believe that stalling was responsible in part for the low turnout as part of a tactic adopted by the Islamist supporters of the constitution," Sabry told The Associated Press.
Judges in charge of polling centers, voters said, took long breaks to eat or pray. Some picked fights with voters who demanded they show documents proving they are indeed judges. Many polling centers closed at 7 p.m., ignoring the election commission's decision to extend voting until 11 p.m.
"Many of the judges closed early on the pretext that they had not received written instructions to extend voting," said Negad Borai, a prominent lawyer and rights activist. "All the problems surrounding the referendum could have been solved if Morsi seriously sought consensus on the draft, rather than ram it through."
Others stayed away in a deliberate boycott of what they called a flawed system manipulated by Islamists.
"The lines were very long. The atmosphere was unsafe," said Nayrouz Abouzid, a 32-year-old magazine publisher and head of a PR firm in Cairo. "Additionally, I felt that by voting I would be legitimizing a process that is both absurd and ridiculous."
Tarek Shalaby, a blogger and a veteran of the anti-Mubarak uprising, pointed to the Constituent Assembly that approved the draft, saying it was not representative.
"If the process of drafting the constitution was appropriate, but the outcome was bad, I would have taken part and said 'no'," he said. "I will not allow them to take me to their court to play their game."