Syria's main opposition groups have joined to create a broad-based national council aimed at unifying their efforts to overthrow President Bashar al-Assad's government, which they accuse of pushing the country to the brink of civil war.

A prominent secular dissident, Bourhan Ghalioun, announced the Syrian National Council's formation Sunday in the Turkish city of Istanbul. While he said the group rejects foreign intervention that “compromises Syria's sovereignty,” Ghalioun appealed to the international community to protect civilians from what he called a government “war” against them.

The Syrian opposition consists of a variety of groups with differing ideologies. The newly formed council includes Syria's pro-democracy Damascus Declaration, the banned Muslim Brotherhood, various Christian and Kurdish factions and the grass-roots Local Coordination Committees, which have led nationwide street protests.

Ghalioun was flanked in Istanbul Sunday by representatives of each group, which include members of the largest Syrian opposition factions. The SNC comprises a general assembly, a general secretariat and an executive committee whose members will chair the council on a rotating basis.

Syria's state news agency said gunmen in the troubled northwest killed the 21-year-old son of the country's top Sunni Muslim cleric in an ambush Sunday. The cleric, Grand Mufti Ahmad Badreddine Hassoun, is considered a close supporter of Mr. Assad's government and has echoed its claims that Syria's unrest is the result of a foreign conspiracy.

Syria has been using military force to crush almost seven months of largely peaceful opposition protests demanding an end to Mr. Assad's 11-year autocratic rule.

The United Nations says at least 2,700 people have been killed in the crackdown. The Syrian government says the dead include hundreds of security personnel killed by what it calls “armed terrorist groups.”

In the latest crackdown on dissent, Syrian government troops regained control of the central town of Rastan on Saturday, after several days of fighting insurgents, including army defectors who joined the opposition. The army defectors retreated from the town ahead of the government takeover.