The Syrian National Coalition has named veteran dissident George Sabra as caretaker leader of the main opposition grouping, following the resignation of Moaz al-Khatib.
Sabra "was assigned today to carry out the functions of the head of the Coalition until elections for a new president," one of the Coalition's main constituent groups, the Syrian National Council, said in a statement on Monday.
Sabra is a leftist, secular opposition figure and a leading member of the Syrian Democratic People's party, a former communist party.
He was a co-founder of the Damascus Declaration opposition coalition in 2005.
Sabra has lived in Syria for most of his life and has been jailed many times for dissent.
In October 2011, he fled to Paris to help form the post-uprising opposition.
'Crime against humanity'
At a press conference in Istanbul following his designation as interim opposition leader, Sabra denounced President Bashar al-Assad's regime for what the opposition has called recent "massacres" outside of Damascus.
The exact death toll in the Jdaidet Artouz and Jdaidet al-Fadel districts could not be confirmed, but Sabra put the number at "more than 500".
The two adjacent neighbourhoods are about 15km southwest of the capital.
"It is beyond description and more barbarian than horror movies," Sabra said.
"What's taking place in Damascus are crimes against humanity," Sabra said. "It is nothing short of genocide, and the international community must act".
The Britain-based Syrian Observatory for Human Rights said the death toll, mostly from shelling, could be as high as 250.
The Local Coordination Committees (LCC), another activist group, put the death toll at 483.
A government official in Damascus told The Associated Press that rebels were behind the "massacre" in Jdaidet al-Fadel, saying they sought to blame government forces who entered the area after the killings.
Jdaidet al-Fadel is inhabited mostly by Syrians who fled the Golan Heights after the area was captured by Israel in 1967.
Jdaidet Artouz has a large Christian and Druse population - two minority communities that have generally stood by Assad or on the sidelines.
Sabra also described Lebanese group Hezbollah's role in fighting in the central Syrian province of Homs as a declaration of war against the Syrian people.
"What is happening in Homs is a declaration of war against the Syrian people and the Arab League should deal with it on this basis," said.
"The Lebanese president and the Lebanese government should realise the danger that it poses to the lives of Syrians and the future relations between the two peoples and countries," he added.
Sabra's condemnation of the role of the Lebanese group follows reports that elite fighters from the organisation were taking the lead in the Syrian regime's battle against rebel fighters in the Qusayr area of Homs.
The area, near the Lebanese border, has been the scene of fierce fighting in recent days, with regime troops capturing a string of strategic villages and raising rebel concerns that the town of Qusayr, an opposition stronghold, could also fall.
"It's Hezbollah that is leading the battle in Qusayr, with its elite forces," Rami Abdel Rahman, director of the Syrian Observatory for Human Rights, told AFP news agency.
The area is considered strategically important because it lies near the Lebanese border and the highway connecting Damascus to the coast.
Assad reportedly told a group of visiting Lebanese politicians at the weekend that the fighting in the area was the "main battle" his forces were waging.
"We want to finish it at any cost," a Lebanese politician at the meeting quoted him as saying.