Leading Muslim preachers condemned Iran and its "Satanic" Shi'ite allies in Friday sermons, after a battle in Syria that has inflamed sectarian rhetoric which risks spreading violence around the Middle East.

In Tehran, the non-Arab power behind the Shi'ite strand of Islam followed by a minority of Muslims, Iran's Supreme Leader Ayatollah Ali Khamenei called for restraint and unity, blaming Western powers and Israel for fomenting the sectarian strife.

But across the Gulf in Saudi Arabia, a bastion of hardline Sunni theology and opposition to Iran, a senior cleric aligned with the US-allied government spoke of a Shi'ite "plot against Islam" that was made newly apparent in the assault by Lebanese Hezbollah fighters on Sunni rebels in the Syrian town of Qusair.

In Qatar, the International Association of Muslim Scholars, a Sunni body headed by influential cleric Sheikh Youssef al-Qaradawi, condemned the "Qusair massacre" and called for "a day of rage" in support of the Syrian people next Friday, according to a statement posted online.

The two-year-old uprising against Iranian-backed Syrian President Bashar al-Assad, whose Alawite minority is an offshoot of Shi'ite Islam, has taken an ever more poisonously sectarian tone as the number of dead and displaced has soared, increasing risks of inflaming the broader Sunni-Shi'ite confrontation.

In Egypt, by far the most populous Arab state, where the Arab Spring protests of 2011 brought the Sunni Islamist Muslim Brotherhood to power, a leading cleric and Brotherhood member led televised prayers on Friday in which he described Hezbollah - 'the party of God' in Arabic - as "the party of Satan".

"God, break the backs of Bashar and his supporters," Salah Sultan, secretary-general of Egypt's Supreme Council for Islamic Affairs, said at a Cairo mosque. "God, break the back of Hezbollah, the party of Satan, God, break the back of Iran."

While Shi'ites and Alawites rejoiced after the border town of Qusair fell on Wednesday to Hezbollah and Assad's troops, talk of vengeance against entire communities has increased both in Syria and across the frontier in Lebanon, where sectarian rivalries remain acute two decades after its own long civil war.

In the northern Lebanese city of Tripoli, where Shi'ites and Sunnis have fought, a hardline cleric, Sheikh Bilal Baroudi, told worshippers: "Hezbollah is responsible for the consequences of this jihad invasion against Sunnis in Qusair.

"The response that is coming will be harsh."

In Beirut, where the Shi'ite southern suburbs erupted in joy after the fall of Qusair, prominent Sunni preacher Da'i al-Islam al-Shahhal urged followers to resist Iranian attempts to control Iraq, Lebanon and Syria as a step to conquering the Gulf states:

"I call on all those zealous and concerned to help us," he said. "Stand with us financially and morally to foil the plan."

Ayatollah Khamenei appealed for pan-Muslim unity against "hegemonic powers" - the "corrupt capitalist" West and Israel: "The enemy's main scheme is to create rifts and conflict among Muslims," he was quoted as saying. "So, unity, solidarity and cooperation are the most pressing needs of the Muslim world."

Senior Saudi cleric Sheikh Saleh al-Fawzan, in comments in al-Madina newspaper, underlined Sunni suspicion of such calls:

Shi'ites, he said, "pretend to be Muslims and try to get closer to the Sunnis ... in order for them to be able to plot against Islam ... these days their hostility has become more apparent in their war against the Sunnis in Syria."

Appeals for a united front, notably against Israel, which once won Hezbollah widespread respect among Sunni Arabs, now fall on deaf ears following the Shi'ite movement's overt drive to save Assad and provide a bulwark for Iranian influence.

As one Lebanese woman in a Sunni neighbourhood of Beirut put it, as she listened with disdain to festivities echoing from a Shi'ite stronghold after Hezbollah's victory at Qusair: "What are they celebrating? You'd think they'd liberated Jerusalem."

Hardliners on each side accuse the other of furthering the goals of common adversaries in the West or in Israel.

In the Gaza Strip, whose Palestinian Hamas rulers were once allies of Assad and Hezbollah, hardline cleric Imad al-Daya told worshippers that Qusair had exposed the "fraud" of Hezbollah's rhetoric about leading "resistance" to Israel.

"Wake up," he told worshippers. "This is a war of religion." Shi'ites, he added, had always been "a knife in Muslims' backs".

The numbers across the Middle East who may be enraged to the point of taking up arms may be small, but any such move threatens to add to the 80,000 or more already killed in Syria and to the countless thousands killed in sectarian bloodletting in Lebanon from 1975 to 1990 and in Iraq over the past decade.

The level of sectarian rhetoric seen from leaders of various kinds in the two weeks since Hezbollah's Sheikh Hassan Nasrallah pledged to fight for Assad until a final victory may pressure regional governments to be seen to take their own hard lines.

The rift among world powers along Cold War lines, between the West and Moscow, has thwarted efforts for a UN peace deal within Syria or to stop the flow of arms to the warring parties.

Israel has already stepped in to the conflict, bombing Syria and now facing trouble on the Golan Heights. Some in the West also now want their governments to intervene to prop up a rebel cause that has suffered a serious setback at Qusair.

Many Sunni clerics have spoken in support of a call to holy war in Syria issued last week by Qaradawi. Based in Qatar, which like Saudi Arabia backs the rebels, he has at times been seen as opposed to favouring reconciliation with Shi'ites.

But he has now said Sunnis should go to fight in Syria. His comments were endorsed by the kingdom's Grand Mufti - whose voice carries special weight in coming from the birthplace of Islam. Similar statements from some of Egypt's ruling Muslim Brotherhood, long at odds with the Saudi religious hierarchy, underlined the broad strength of feeling.

Many mainstream mosques in Egypt avoided outspoken comment on Syria on Friday. Islamist President Mohamed Mursi has opened some rapprochement with Tehran in the past year. But the arrival of Iranian tourists that this warming of relations has brought since the fall of the US-backed rule of Hosni Mubarak has been accompanied by new criticism of Shi'ites by some in Egypt.

In the Red Sea city of Hurghada, where some Iranians have been on holiday lately, preacher Mohamad Daraz, whose Salafist movement sees the Brotherhood as too liberal, said: "God, annihilate the Shi'ites and those who cooperate with them."

While there are few Shi'ites in Egypt, risks of sectarian conflict have risen from the Gulf in the east, where Saudi Arabia has a Shi'ite minority, through Iraq and Syria to Lebanon and Turkey in the west.

Syria and its immediate neighbours Lebanon and Iraq provide a proxy battleground for bigger regional powers that seem, for now, unlikely to risk a direct conflict. But there is also little sign of leaders trying to moderate sectarian passions.

In a blog for Foreign Policy magazine, Marc Lynch of George Washington University saw the hard line taken by the sometimes conciliatory Qaradawi as a signal that Sunni leaders may try to outbid each other, raising the stakes of anti-Shi'ite populism.

"He clearly calculates that anti-Shi'ite sectarianism in support of the Syrian insurgency is both strategically useful and a political winner," Lynch wrote. "Expect the competition among Sunnis to adopt the most extreme stances to accelerate.

"By the time more responsible figures realise the destructive forces they've unleashed - or Qaradawi attempts his standard pivot towards reconciliation - it may be too late," Lynch added.-Reuters