Egyptian Islamists plan big rally as referendum looms

* Islamists to stage rally in Alexandria

* Second day of voting on constitution on Saturday

* Measure expected to pass after first-round "yes" vote

CAIRO, Dec 21 (Reuters) - Egyptian Islamists are planning a mass protest in Alexandria on Friday in a move likely to raise tensions on the eve of a divisive referendum that will determine the political future of the Arab world's biggest nation.

The Muslim Brotherhood called for the rally after a violent confrontation between Islamists and the liberal, secular opposition in Egypt's second city last week ended with a Muslim preacher besieged inside his mosque for 14 hours. Rival factions were armed with clubs, knives and swords.

The run-up to the referendum on a new draft constitution has been marked by often violent protests in which at least eight people have died.

The constitution is backed by President Mohamed Mursi and his Islamist allies 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.

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.

"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 in a statement.

POLL WITHIN TWO MONTHS

The constitution must be in place before elections can be held. If it passes, the poll should be held within two months.

Mursi and his backers say the constitution is needed to advance Egypt's transition from decades of military-backed autocratic rule. Opponents say it ignores the rights of women and of minorities, including the 10 percent of Egyptians who are Christian.

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 was 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.