The vote Saturday is expected to draw 25.5 million voters in 17 governates, about half of Egypt's eligible voters, in the second phase of the national referendum on the country's first post-uprising constitution, the news website Ahram Online reported Friday.
The first phase of the referendum Dec. 15 -- in 10 governorates -- had about 57 percent voting in favor of the draft, unofficial results indicated.
The opposition says the constitution gives Islamists too much power, ignores personal freedoms and disregards the rights of women and minorities, including Coptic Christians, who make up 10 percent of Egypt's population.
Islamic protesters included the Muslim Brotherhood, the powerful Islamist organization Morsi once led, along with his Freedom and Justice Party, the Muslim Brotherhood's political arm, the Muslim Brotherhood said in a statement, as well as several ultraconservative political movements, the organization said.
The main opposition coalition, the National Salvation Front, led by Nobel Peace Prize laureate and former International Atomic Energy Agency chief Mohamed ElBaradei, didn't call for a counter-demonstration but reiterated its call for a "no" vote in Saturday's referendum.
"For the sake of the future, the masses of our people should strongly and firmly say 'no' to injustice and 'no' to the Brotherhood's dominance," the Front said a statement.
A rally in Alexandria, Egypt, turned violent Friday and police used tear gas to break up rival political groups.
Security forces took the action to control crowds at the Qaed Ibrahim Mosque as separate groups of protesters threw stones during a demonstration called to "defend [Islamic] scholars and mosques" and to call for Shariah law, the legal code of Islam, Ahram Online reported.
Human right activists said 49 people were beaten, arrested and detained for up to 17 hours during deadly clashes Dec. 5 near Cairo's presidential palace, allegedly by Morsi's supporters.
Despite allegations the detainees admitted in confessions they were "paid thugs," a local prosecutor released them for lack of incriminating evidence, CNN reported Friday.