At least 19 people were reported injured in the violence in Alexandria, which broke out after an ultraconservative cleric urged worshippers to vote "yes" and described the opposition as "followers of infidels."
The crisis pits Egypt's newly empowered Islamists against the country's mainly liberals and supported by a large sector of apolitical moderate Muslims. Both sides stepped up their campaigns after weeks of violence and harsh divisions that have turned Saturday's vote into a fight over Egypt's post-revolutionary identity.
Critics already have raised concern over the legitimacy of the document after most judges said they would not supervise the vote as stipulated by law. Rights groups voiced fears of fraud and the opposition said a decision to stretch the vote two rounds to make up for the shortage of judges left the door open for initial results to sway voter opinions.
On Friday, thousands of Islamists filled a square in northern Cairo, raising pictures of President Mohammed Morsi, who has insisted the referendum will begin on Saturday as scheduled despite accusations the entire process has been rushed. A few kilometers (miles) away, the opposition chanted for a "no" vote in a sit-in, exposing the deep rifts.
Religious authorities had issued orders that mosques should not be used to manipulate the vote, but several clerics, especially in conservative southern areas, took to the pulpit to tell their congregations that voting in favor of the constitution is seeking victory for Islam.
"Voting yes is like jihad for the sake of God," Sheik Abdel-Akher Hamad told worshippers in the southern city of Assuit. "It preserves Egypt from evils and from those who want to sabotage Islam and Muslims."
The crisis began when Morsi, who hails from the Muslim Brotherhood, issued a decree on Nov. 22 giving himself and the assembly writing the draft immunity from judicial oversight so the document could be finalized before an expected court ruling dissolving the panel. The panel has been disbanded before over illegality of its composition since it included lawmakers.
On Dec. 1, the densely written document was then passed by an 85-member assembly mostly composed of Islamists in a marathon session despite a walkout by secular activists and Christians. Morsi rushed it to a vote scheduled for the next two Saturdays, compounding the crisis.
Many also fear that it would be next to impossible to amend the constitution if the draft passes and Islamists continue to dominate the parliament in upcoming elections.
According to the draft, articles 217 and 218 state that the president and parliament have the right to make a "request" to "amend an article or more," then parliament must discuss the request within 30 days. Two thirds of parliament members are needed to pass the request. Then parliament has 60 days to finalize the amended articles, and a third of parliament is needed to pass the final text before putting them to a national referendum.
Most of Egypt's judges are refusing to monitor the vote, according to the powerful Judges union, although authorities said they would be able to meet the legal obligation to have a judge at each polling station.
More than 51 million people are registered to vote, with more than 6,000 polling stations in 10 provinces, including Cairo and Alexandria in the first round on Saturday.
"Polling stations can't open their doors unless there is a judge there," Zaghloul el-Blashi, the head of the referendum committee, told the pan-Arab TV station Al-Jazeera.
The Carter Center, the international group founded by former U.S. President Jimmy Carter that has been monitoring Egyptian voting since last year's uprising, also said it would not deploy monitors for the referendum because of the government's late release of monitoring regulations.
The opposition coalition The National Salvation Front reiterated its call for Morsi to postpone the referendum and form a new assembly to draft a new constitution.
"History will remember that this regime forced a referendum on the people of Egypt in these harsh circumstances," said Ahmed Said, leader of liberal Free Egyptians Party and part of the coalition opposition. "They can't find judges to monitor, (there is a) rift among Egyptians and blood on the streets."
The crisis also has united the long fractured opposition, which had considered boycotting the referendum but instead officially launched a campaign Thursday calling on voters to go to the polls and reject it. They voiced fears of fraud.
"We know well that the rigging operation is beginning from now. There are creative ways. They are counting on the lack of awareness among the people," Sameh Ashour, an opposition member, said Friday.
Clashes broke out in at least two cities after fiery sermons promoting a vote for the constitution as a vote for Islam.
Sheik Osama el-Hawi, a member of the ultraconservative Islamic sect known as Salafis, told worshippers approval of the constitution was the only way to restore stability after nearly two years of turmoil following the revolution that ousted Hosni Mubarak.
"Saturday will be the day of victory for Shariah (Islamic law)," he said.
His followers then briefly fought with protesters marching outside the mosque after prayers in Nagaa Hammadi, 460 kilometers (290 miles) south of Cairo.
In Alexandria, witness Mustafa Saqa said Sheik Ahmed el-Mahalawi, a well-known Salafi, gave a sermon in which he described the opposition as "followers of infidels" sparking tension that quickly devolved into fist fights and street battles that spread into residential areas. State TV showed footage of Islamists brandishing swords as protesters hurled rocks at each other.
A health official, speaking on condition of anonymity because he wasn't authorized to release the information, said at least 19 people were injured.
Morsi, who took office in June after a narrow victory in the country's first free elections, joined weekly Friday prayers at the el-Farouk mosque near his house in eastern Cairo and left without giving a speech.
The cleric at his mosque remained neutral.
"Those who think that rejecting or approving the constitution is the path to heaven or hell is mistaken," he said, referring to a slogan constitution supporters have used. "No one rules whether someone goes to heaven or hell but God almighty."
On Friday, Islamist members of the assembly that drafted the constitution held a presser on Friday in which they accused opposition of spreading "lies and calls for discord."
"This is political blackmail that is not based on any evidence or reality," said Amr Darrag, a leading member of the Muslim Brotherhood's political arm Freedom and Justice.
But in many ways, the pros and cons of the draft constitution have been overshadowed by the worst crisis to hit Egypt since the overthrow nearly two years ago of Hosni Mubarak's authoritarian regime. Newspaper and TV commentators have warned of a country moving toward civil strife and a schism that may not be bridged.
Abroad, expatriates were voting in Egyptian diplomatic missions on Friday for the third day
The opposition took out advertisements in newspapers and on television detailing their arguments against the charter. The Morsi camp's message was far simpler. A "Yes" to the constitution is a yes to Islam.
Sheik Mohammed Sayyed, affiliated with the Muslim Brotherhood, put it bluntly during prayers at the el-Helali mosque elsewhere in Assuit. "Tomorrow is the day we will seek victory of Islam," he said.
"The first phase of implementing Shariah (Islamic law) is the election of a Muslim president. The second phase is to hold referendum on the constitution," he said, urging voters to go to the polls in groups. "Those calling themselves liberals and salvation of Egypt, are saboteurs who sabotage Egypt."