By Crispian Balmer , BEIRUT (Reuters) – A Lebanese Sunni preacher lead thousands of anti-government Shi’ite protesters in Friday prayers at a tent camp in central Beirut in a show of Muslim unity.

The opposition is escalating a protest campaign to topple the government of Prime Minister Fouad Siniora despite fears of sectarian strife.Shi’ite Hezbollah one of the most powerful force in the opposition while Siniora is a Sunni and his main backer, parliamentary majority leader Saad al-Hariri, is also a Sunni.Preacher Fathi Yakan, who leads a small pro-opposition Sunni group, will lead the noon prayers. Hezbollah chief Sayyed Hassan Nasrallah urged supporters on Thursday to attend the sermon to display Muslim unity.

"There will not, will not, be fighting between Shi’ites and Sunnis in Lebanon," Nasrallah told tens of thousands of supporters in central Beirut via a live video link.Nasrallah vowed to pursue his demand for a government of national unity and told Siniora to agree swiftly or face the prospect of new opposition demands for a transitional administration leading to early parliamentary elections.

In a scathing attack, Nasrallah accused members of the anti-Syrian majority coalition of lobbying Washington to get Israel to attack on Lebanon last July to crush Hezbollah.

He also said Siniora had tried to get the Lebanese army to cut supply routes to his guerrillas during the 34-day war.

Government sources denied the accusations.

The opposition, which includes a Christian party, has paralyzed the heart of Beirut for a week in an around-the-clock anti-government protest that shows no sign of fading.

Speaking in New York, U.N. Secretary-General  Kofi Annan on Thursday urged all sides to resume negotiations.

Annan remained "very concerned" about Lebanon, said his chief spokesman Stephane Dujarric, and had called for "a political solution to the current impasse."

Some commentators have said the tense political stand-off could degenerate into widescale violence in a country that has suffered two civil wars in the past 50 years.

The protests have already ignited several sectarian clashes between Shi’ites and Sunnis in Beirut, with one Shi’ite demonstrator shot dead in a Sunni neighborhood last Sunday.

Siniora allies say their opponents are looking to derail plans to set up an international tribunal to try suspects in the 2005 assassination of former Prime Minister Rafik al-Hariri, which many Lebanese blame on Syria, a charge Damascus denies.

The army, which has around 45,000 soldiers and officers, split along sectarian lines in the 1975-1990 civil war.