After days of wrangling in Qatar under constant cajoling by exasperated Arab, US and other officials, representatives of groups including rebel fighters, veteran dissidents and ethnic and religious minorities agreed on Sunday to join a new assembly that can form a government-in-exile. They unanimously elected reformist Damascus cleric Mouaz al-Khatib as its president.
Khatib, a soft-spoken preacher who was once imam of the ancient Umayyad Mosque in Damascus, immediately called on soldiers to quit the Syrian army and on all sects to unite.
"We demand freedom for every Sunni, Alawi, Ismaili (Shi'ite), Christian, Druze, Assyrian ... and rights for all parts of the harmonious Syrian people," he told reporters.
It remains to be seen whether the Syrian National Coalition for Opposition and Revolutionary Forces can overcome the mutual suspicions and in-fighting that have weakened the 20-month-old drive to end four decades of rule by President Assad's family.
But for allies who see it emulating Libya's Transitional National Council, the deal was welcome on a day when Israel fired a missile after a Syrian mortar bomb hit the Golan Heights and Assad's air force strafed along Turkey's border.
"We will strive from now on to have this new body recognised completely by all parties ... as the sole legitimate representative of the Syrian people," said Prime Minister Sheikh Hamad bin Jassim of Qatar, an important supporter of the rebels.
Turkish Foreign Minister Ahmed Davutoglu said there was "no excuse any more" for foreign governments not to support an opposition whose internal divisions had given many pause.
The United States had also strongly promoted the plan for the Doha meeting to unite the various factions and, notably, subsume the previously ineffectual Syrian National Council into a wider body that would be more inclusive of minorities from a country of great ethnic and religious diversity.
France, a vocal backer of the rebels and which once ruled Syria, hailed the deal. "France will work with its partners to secure international recognition of this new entity as the representative of the aspirations of the Syrian people," Foreign Minister Laurent Fabius said in a statement in which he called the Assad government "the criminal regime in Damascus".
Twenty months after street demonstrations inspired by the Arab Spring drew a military response from Assad, his enemies hope a more cohesive opposition can break a stalemate in the civil war and win more military and diplomatic support from allies who have been wary of the influence of anti-Western militants, some of them linked to al Qaeda.
While there has been renewed talk in Turkey and elsewhere of providing some sort of no-fly zones or other protection for refugees and the lightly armed rebels facing Assad's air force, Western governments have shown little appetite for new military ventures in such a complex Arab state.
And Russia and China, which have blocked previous moves against Assad in the United Nations Security Council, are unlikely swiftly to alter positions which call for dialogue with Assad and view opposition groups as being in thrall to the West.
Regional power Iran, in whose Shi'ite brand of Islam Assad's Alawite minority has its religious roots, remains firmly behind the president in a conflict which pits him against majority Sunni Muslims supported by Iran's Sunni Arab adversaries.
After long arguments over whether and how to form the new opposition assembly, the speed with which a consensus emerged within hours to ensure that Khatib stood unopposed for the post of president was notable and may encourage its supporters.
His deputies will be Riad Seif, a veteran dissident who had proposed the U.S.-backed initiative to set up an umbrella group uniting groups inside and outside Syria, and Suhair al-Atassi, one of the relatively few women with a leading role. Delegates said a third deputy may yet be named from among ethnic Kurds.
Businessman Mustafa Sabbagh was elected general secretary.
Khatib, 50, was jailed several times for criticising Assad. He finally fled into exile this year.
"This is a serious step against the regime and a serious step towards freedom," Syrian National Council leader George Sabra said of Khatib, who has long promoted a liberal Islam tolerant of Syria's Christian, Alawite and other minorities.
Critics of the SNC had said it was too much influenced by the Sunni Islamist Muslim Brotherhood and too little open to minorities, including Alawites, some 10 percent of the population who fear a backlash if Assad is overthrown after a war that has taken on increasingly sectarian characteristics.
SNC member Wael Merza said all Assad's opponents were now welcome. "We are open to all the real opposition powers that have weight, influence and the same aims as the Coalition to bring down the regime and establish a democratic Syria."
In a sign of the wider sectarian confrontation across the Middle East, three people were killed on Sunday in the Lebanese coastal city of Sidon in fighting between Sunni Islamists and the Iranian-backed Lebanese Shi'ite movement Hezbollah.
In the Golan Heights, Israeli troops fired a guided missile into Syria on Sunday in a potent "warning shot" after mortar fire from fighting between Syrian troops and rebels hit the Israeli-occupied territory for the second time in four days.
Israel Radio said it was the first direct engagement of the Syrian military on the Golan since the war of 1973. There was no immediate comment from the 1,000-strong United Nations force which patrols the area, and no reaction from Syria.
In other violence, Assad's troops bombarded the Ras al-Ain area on the border with Turkey, days after the town fell to rebels during an advance that has sent thousands of refugees fleeing for safety.
Increasingly critical of the failure of world powers to halt the war, Turkey is in discussions with its NATO allies over the possible deployment of Patriot surface-to-air missiles to defend against any spillover of violence. The move could also be a step towards enforcing a no-fly zone within Syria.
More than 38,000 people have been killed and many tens of thousands more displaced in the violence since March last year.