Syria's newly named opposition leader Mouaz Alkhatib, a soft-spoken cleric, is being backed by the US and Gulf Arab states.
He has launched his quest for international recognition of a government-in-waiting to topple President Bashar al-Assad.
Western and Arab enemies of President Assad hope the creation of a new Syrian National Coalition for Opposition and Revolutionary Forces can finally unify a fractious and ineffective opposition.
Mr Alkhatib,50, a former imam of a Damascus mosque, flew to Cairo to seek the Arab League's blessing for the new assembly, the day after he was unanimously elected to lead it.
He made a concerted effort to address the sectarian and ethnic acrimony underlying 20 months of civil war that has killed 38,000 people.
The Arab League welcomed the formation of a new Syrian opposition group, but stopped short of giving it full recognition as the representative of the Syrian people.
Some Arab states are still reluctant to completely abandon Syrian President Bashar al-Assad.
Many Arab League foreign ministers meeting in Cairo were unable to state clearly that the new Syrian National Coalition was the sole legitimate Syrian voice.
Reaction to the SNC
The SNC, dominated by the Qatar-backed Brotherhood, agreed under intense US and Qatari pressure to become a minority player in a wider body, the Syrian National Coalition.
The new body will try to win international recognition as the sole legitimate representative of the Syrian people.
It will become the main address for political, humanitarian and military support for the revolt against President Bashar al-Assad.
Many prominent figures resigned from the SNC in recent months complaining that Islamists were sidelining minorities and women.
Another three left in Doha, including SNC founder Adib Shishakly, angered over the failure of women to make it onto the SNC's new general secretariat elected last week.
Meanwhile in the US, with electoral pressures gone, a major deployment of US troops remains unthinkable.
The kind of more limited but sustained air campaign that helped oust Libya's Muammar Gaddafi is also off the table, at least for now.
As the body count has mounted in Syria, there have been growing signs the war is also destabilising neighboring states, particularly Lebanon but also Iraq, Turkey and Jordan.
Israel fire shells into Syria in retaliation
Elsewhere, The Israeli military said its tanks fired shells into Syria in response to a mortar round that struck the Golan Heights.
It was the second time in two days that Israel has responded to what it said was errant Syrian fire.
Syria and Israel have not fought over the Golan since the 1973 Middle East conflict, but are still formally at war.
At the northern end of Syria, refugees fled to Turkey when Syrian jets bombed a rebel-held town of Ras al-Ain close to the Turkish border.
The Syrian Observatory for Human Rights said 12 people, including seven Islamist militants, had been killed in the air strikes on Ras al-Ain, which fell to rebels last week during an advance into Syria's mixed Arab and Kurdish northeast.
Another opposition group put the Ras al-Ain death toll at 16.
The pro-opposition observatory, which tracks the violence from Britain, said 140 people were killed in Syria on Sunday.
More than 38,000 people have been killed since March last year.
Turkey, whose border security worries were heightened by a sudden influx of 9,000 refugees within 24 hours last week, has consulted its NATO allies about possibly deploying Patriot surface-to-air missiles to deter Syria's air force.
Such a move could be a prelude to enforcing a no-fly zone in Syria, although Western powers have fought shy of this.