* Islamists hold rally in Alexandria
* Heavy security at Friday prayers
* Second day of voting on constitution on Saturday
* Measure expected to pass after first-round "yes" vote
By Tamim Elyan
ALEXANDRIA, Egypt, Dec 21 (Reuters) - Thousands of chanting Islamists rallied on Friday in support of an Islamic vision of Egypt's future on the eve of a constitutional referendum that has divided the most populous Arab nation.
The Muslim Brotherhood called for the mass gathering in Alexandria to protest after a violent confrontation between Islamists and the liberal, secular opposition last week ended with a Muslim preacher besieged inside his mosque for 14 hours. Rival factions had used clubs, knives and swords.
The run-up to the second and final round of voting on a new constitution on Saturday has been marked by often violent protests that have cost at least eight lives. The first round on Dec. 15 produced a "yes" vote that is expected to be repeated in the second round.
Islamists formed groups checking worshippers arriving for Friday prayers at Alexandria's al-Qaid Ibrahim mosque, scene of last week's violence.
Lines of riot police cordoned off the mosque, where Islamists chanted pro-Islamic slogans while a smaller group of opposition protesters gathered nearby, chanting against Islamist President Mohamed Mursi.
"The people want the implementation of sharia," the Islamist sympathizers shouted, in a show of support for Islamic law. "Our souls and blood, we sacrifice to Islam," they shouted.
In a sign of how high tensions were running, an Islamist supporter filming the anti-Mursi protesters was grabbed and roughed up. Islamists on the other side of a security cordon pushed and shoved police in a bid to get to the man to help him.
Mursi and his Islamist allies back the draft constitution as a vital step in Egypt's transition to democracy almost two years after the fall of Hosni Mubarak.
The opposition, facing defeat in the referendum, has called for a "no" vote against a document it views as leaning too far towards Islamism. They say it ignores the rights of women and minorities, including the 10 percent of Egyptians who are Christian.
Opposition supporter Ali al-Banna, a 51-year-old businessman, said: "We reject the constitution. Mursi's legitimacy has collapsed and we will bring him down."
The first day of voting on Dec. 15 resulted in a 57 percent majority in favour of the constitution. The second stage on Saturday is expected to produce another "yes" vote as it covers regions seen as more conservative and likely to back Mursi.
The National Salvation Front, the main opposition coalition, said a "no" vote meant taking a stand against attempts by the Muslim Brotherhood, Mursi's political base, to dominate Egypt.
The constitution must be in place before elections can be held. If it passes, the poll should be held within two months.
Demonstrations erupted when Mursi awarded himself sweeping powers on Nov. 22 and then fast-tracked the constitution through a drafting assembly dominated by his Islamist allies and boycotted by many liberals.
The referendum is being held over two days because many of the judges needed to oversee polling stayed away in protest. In order to pass, the constitution must be approved by more than 50 percent of those voting.
Adding to the uncertainty as the final round of the referendum approaches, Egypt's chief prosecutor suddenly announced that he was retracting his decision to quit.
Prosecutor Talaat Ibrahim, appointed by Mursi when he assumed his new powers, said he had changed his mind because his resignation on Monday had been under duress.
Ibrahim had quit after more than 1,000 members of his staff gathered at his office to demand he step down because his appointment by the president, rather than by judicial authorities, threatened the independence of the judiciary.
After he announced he was staying, several prosecutors announced they were suspending work and would stage an open-ended protest outside Ibrahim's office.