Syrian opposition figure Haytham al-Maleh told reporters on Tuesday that he has been tasked with forming a government in exile based in Cairo.
"I have been tasked with leading a transitional government," Maleh said, adding that he will begin consultations "with the opposition inside and outside" the country.
Maleh, a conservative Muslim, said he was named by a Syrian coalition of "independents with no political affiliation".
More than 20,000 people have been killed in Syria since a revolt against President Bashar al-Assad's rule began in March 2011, according to the Syrian Observatory for Human Rights. There is no way to independently verify the figure, while the UN has stopped keeping count.
There have been repeated calls on Assad to step down.
When that happens, "we don't want to find ourselves in a political or administrative vacuum," Maleh said.
"This phase calls for cooperation from all sides," he said.
Maleh, 81, is a Syrian laywer and human rights activist who has spent several years in prison in his homeland.
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He was jailed in October 2009 and released in March 2011 by presidential pardon, just days before the revolt against Assad erupted.
Maleh has worked for Amnesty International since 1989 and helped found the Syrian Association for Human Rights.
He was also imprisoned in 1980 for six years along with a number of trade unionists and political dissidents.
There have been other attempts by the Syrian opposition to prepare for a post-Assad future.
On Monday, Syrian rebels distributed what they called a "national salvation draft" proposal for a political transition, bringing together military and civilian figures.
The draft by the joint command of the rebel Free Syrian Army (FSA) proposes the establishment of a higher defence council charged with creating a presidential council, which in turn would bring together six military and civilian figures to lead a future transition.
The proposal "meets all the revolution's demands," said the umbrella Military Council Joint Command, based in the central province of Homs.
When Syria's uprising first turned into an armed insurgency, rebel factions had little or no coordination with each other as they separately battled Assad's forces.
Over time, the Joint Command, headed by Colonel Kassem Saadeddine, has emerged as an increasingly influential coordinating body among rebel commanders inside Syria.