By Alastair Macdonald and Tom Perry

CAIRO, June 28 (Reuters) - Egypt's leading religious authority warned of "civil war" on Friday and called for calm after a member of the ruling Muslim Brotherhood was killed ahead of mass rallies aimed at forcing the president to quit.

"Vigilance is required to ensure we do not slide into civil war," the Al-Azhar clerical institution said in a statement reported by state media. It blamed "criminal gangs" who attacked mosques for street violence. Clashes linked to the political tensions have killed five and wounded scores in recent days.

The Brotherhood said all those killed were Mursi supporters, though this could not be independently verified.

The ancient Cairo academy, which traditionally maintains a distance from the political establishment, also urged opponents of Islamist President Mohamed Mursi to accept his offer of dialogue rather than pressing on with plans for demonstrations.

Welcoming an offer by Mursi on Wednesday to include the fragmented opposition in committees to review the constitution and promote national reconciliation, senior Al-Azhar scholar Hassan El-Shafei said they should accept "for the benefit of the nation instead of the insistence on confrontation".

Opposition leaders dismissed Mursi's offer as a repeat of suggestions they say have gone nowhere because the Brotherhood refuses to dilute its power.

Their supporters will gather in Cairo's Tahrir Square, site of the 2011 revolution, and in other cities on Friday and plan mass rallies on Sunday, when Mursi will complete his first year in office. The Brotherhood will gather supporters after Friday prayers near a mosque in northern Cairo to show their strength.

The movement said one of its members was shot dead and four wounded in an attack on a provincial party office in the Nile Delta city of Zagazig overnight and blamed anti-Mursi activists, which it portrays as an alliance of liberals and loyalists of ousted dictator Hosni Mubarak.

The army has urged both sides to reconcile and has warned that it could step back in to impose order if violence spins out of control - though it insists it will defend the democracy born out of the uprising against Mubarak in early 2011.

Mursi and the Brotherhood accuse loyalists of the old regime of being behind violence and of thwarting their efforts to reform an economy hobbled by corruption. Opponents accuse the Islamists, who have won a series of elections against a diffuse opposition, of seeking to entrench their power and impose Islam.

In a speech on Wednesday, Mursi denounced his critics but admitted some mistakes and offered talks to ease polarisation in politics that he said threatened Egypt's new democratic system.

But opposition leaders said their protests would go ahead.

"Dr. Mohamed Mursi's speech of yesterday only made us more determined in our call for an early presidential vote in order to achieve the goals of the revolution," the liberal opposition coalition said after its leaders met to consider a response.

"We are confident the Egyptian masses will go out in their millions in Egypt's squares and streets on June 30 to confirm their will to get the Jan. 25 revolution back on track."

CROWDS

With the start of Egypt's weekend, people began to gather in Cairo's Tahrir Square, site of the uprising of Jan. 25, 2011, and at venues in other towns. The atmosphere was largely festive but there were widespread fears of trouble in the days ahead.

It is hard to gauge how many may turn out but much of the population, even those sympathetic to Islamic ideas, are deeply frustrated by economic slump and many blame the government.

Previous protest movements since the fall of Mubarak have failed to gather momentum, however, among a population anxious for stability and fearful of further economic hardship.

The army, which helped protesters topple Mubarak and is on alert across the country guarding key locations, says it will act if politicians cannot reach consensus. The United States, which continues to fund the military as it did under Mubarak, has urged Egypt's leaders to pull together.

MEDIA

In his speech, Mursi threatened legal action against several named prominent figures. He said some judges and civil servants were obstructing him, and accused liberal media owners of bias.

Hours after he publicly accused one TV channel owner of tax evasion, the businessman, Mohamed al-Amin, found he was under investigation and barred from leaving the country, prompting his lawyer to tell Reuters: "This is dictatorship." Amin's channel notably airs satire modelled on that of U.S. comic Jon Stewart.

Separately, officials ordered the arrest of a talk show host on another channel known for his anti-Islamist diatribes and ordered that station to be shut down for inciting mutiny in the army and for insulting the armed forces and the police.

An anchor on state television resigned dramatically, live on air, in protest at what he said were attempts by the information minister, an Islamist, to control his programme.

Instability in the most populous Arab nation could send shocks well beyond its borders. Signatory to a key, U.S.-backed peace treaty with Israel, Egypt also controls the Suez Canal, a vital link in global transport networks between Europe and Asia.

"Egypt is historically a critical country to this region,"

U.S. Secretary of State John Kerry, who is on a tour of the Middle East, said on Wednesday, highlighting economic problems.

"Our hopes are that all parties ... the demonstration that takes place on Friday or the demonstration that takes place on Sunday, will all engage in peaceful, free expression," he said.

With the government short of cash and seeking funding from allies and the IMF, Kerry said Egyptshould curb unrest in order to attract investment and restore vital tourism income. The U.S. ambassador in Cairo has angered opposition activists by saying explicitly that their protests risked being counter-productive.

(Reporting by Shaimaa Fayed, Patrick Werr, Asmaa Alsharif, Tom Perry, Maggie Fick, Yasmine Saleh, Omar Fahmy, Alexander Dziadosz, Omar Fahmy and Shadia Nasralla in Cairo and Abdelrahman Youssef in Alexandria; writing by Alastair Macdonald; editing by Philippa Fletcher)