Wednesday's call for a "no" vote followed a prolonged debate within the opposition over whether to boycott the referendum a threat that still hung in the air as the anti-Morsi camp laid down its conditions for participation.
These included full judicial supervision, independent and international monitors, and adequate security. If they were not met, Morsi's opponents said, the opposition would call a last-minute boycott.
On Tuesday, the vast majority of Egypt's judges rejected any role in overseeing the referendum.
Egypt's crisis began on Nov. 22 when Morsi issued decrees, since rescinded, that placed him above judicial oversight. At the heart of the standoff now is the draft charter, which Morsi's opponents contend allows religious authorities too much influence over legislation, threatens to restrict freedom of expression and opens the door to Islamist control over day-to-day life
The fallout has left Egypt the most divided it has been since Hosni Mubarak's ouster nearly two years ago. It pits Morsi, his Muslim Brotherhood and ultraconservative Islamists on one side, against the rest of the country, including liberals, leftists and Christians, on the other.
On Wednesday, Hamdeen Sabahi of the National Salvation Front, the main opposition group, urged followers to "topple the constitution by voting 'no'" on Saturday.
"The constitution is a decisive battle but not the final one," said Ahmed Khairi of the opposition Free Egyptians party. "We will keep on fighting for our demands and for Egypt to become a country for all. This will not be the end."
The opposition's chances of overturning the charter hinge on whether it can bring out its supporters to the polls. The Islamists are disciplined voters and have portrayed a "yes" vote as one for Islam and a "no" vote as one for immorality.
An Islamist-dominated panel rushed through the draft constitution in a marathon session last month. Islamists say its approval will restore political stability and allow the rebuilding of state institutions.
The nationwide referendum was initially scheduled to take place on Dec. 15, but on Tuesday, Morsi ordered the voting stretched into another day on Dec. 22. Voting must be overseen by judges and their absence could cast doubt on the legitimacy of the vote and thus the constitution itself.
Zaghloul el-Balshi, head of the referendum's organizing committee, said on Tuesday that 9,000 judges had agreed to oversee the voting, though his claim could not be independently verified.
Egypt has nearly 13,000 polling station, each of which normally requires a judge. Aides to Morsi have said judges are only needed to supervise the 9,000 main stations, while government employees or university lecturers can fill in at the rest.
The start of overseas voting after nearly three weeks of opposition protests showed Morsi's determination to go forward with the process.
The vote by half a million expatriates overseas could give hints about which way the referendum is going. Egyptian expatriates in the Gulf are known to lean toward the Islamists, while those in Europe, North America and Australia, among them a large number of Christian migrants, lean more toward the liberals.
"We are against what is happening in Egypt nowadays. I am not against Mohamed Morsi, but there are things happening and we need to take a decision before the beginning of a war in Egypt," said Hakim Ousama, an Egyptian living in Paris, as he prepared to cast his vote.
Islamists who support the draft constitution have been distributing flyers and posters to urge a "yes" vote, and have used mosques to get out their message. The opposition, for its part, has launched ad campaigns on independent TV networks, featuring the catch phrase: "I don't approve of the constitution that divides us."
In a move likely to stoke the judges' anger, Egypt's top prosecutor, Morsi appointee Talaat Abdullah, removed the judge in charge of an investigation into violence outside the presidential palace last week that began when Islamists loyal to Morsi set upon opposition protesters staging a sit-in.
The judge, Mustafa Khater, had ordered the release from detention of most suspects for lack of evidence, a move that drew criticism from Mohammed Badie, the influential head of Morsi's Muslim Brotherhood.
Human Rights Watch, meanwhile, called on authorities to investigate the detention and abuse of opposition protesters by Brotherhood supporters during last week's clashes.
In another twist, Egypt's military withdrew a call for talks with the opposition, one day after proposing it.
Defense Minister Abdel-Fatah el-Sissi decided to postpone the talks because "the response to the invitation was below expectations," military spokesman Col. Ahmed Mohammed Ali was quoted by the official MENA news agency as saying. He did not elaborate.
That announcement came just as the opposition said it was willing to attend the meeting. El-Sissi's call was the second time in less than a week that the generals addressed the crisis. On Saturday, the military warned of "disastrous consequences" if the crisis is not resolved.
The cancellation was likely made under pressure from Morsi, who has been adamant that the military must stick to its core security mission.
With the military now weighing in, Egypt may face further divisions.
"The military is boiling. The military doesn't live in isolation from the street and what is happening there. We have judges on strike, a constitutional court under siege," said retired army general Hossam Sweilam, a military analyst who is widely thought to be close to the military leadership.
"We have large sectors of Egyptians rejecting the referendum and we have so many ways to postpone it. But the stubborn leadership is insisting on going forward with the process. All this is reflected on the armed forces," he said.