* Former Prime Minister Hijab withdraws his candidacy
* Independents hold sway vote -coalition sources
* Muslim Brotherhood casts large shadow over process (Adds quotes, voting blocs, background)
By Khaled Yacoub Oweis
AMMAN, March 7 (Reuters) - The opposition Syrian National Coalition will meet in Istanbul next week to elect a provisional prime minister to run a political transition if President Bashar al-Assad falls, coalition members said on Thursday.
The opposition wants to create a provisional government to administer rebel-held areas in Syria and to show it can fill a power vacuum left by collapsing state institutions, putting structures in place to curb chaos in a post-Assad era.
The Istanbul meeting on March 12 and 13 was called after former Prime Minister Riad Hijab, the highest-ranking civilian defector from President Bashar al-Assad's government, withdrew his candidacy, several coalition members said.
Hijab had run into opposition from Islamists and liberals in the coalition for his past ties with Syria's ruling hierarchy.
"The field of candidates has been expanding since Hijab withdrew," said one coalition member who asked not to be named.
The opposition had planned to meet on March 2, but postponed the talks partly because of Hijab's withdrawal, he said.
Hijab said in a statement he had informed the coalition's president, Moaz Alkhatib, that he would not join the provisional government after talks in Cairo last month, but gave no reason.
Coalition sources said the Syrian National Council, a large Muslim Brotherhood-influenced bloc within the 71-member coalition, had chosen three candidates for prime minister.
They are Salem al-Muslet, a tribal figure from northeastern Syria who worked at think-tanks in the Gulf; Osama al-Qadi, a U.S.-educated economist who heads an opposition taskforce drawing up plans for post-conflict economic recovery; and veteran opposition campaigner Burhan Ghalioun, a professor from Homs and previous president of the Syrian National Council.
Asaad Mustafa, a former agriculture minister during the 30-year rule of Assad's father, the late President Hafez al-Assad, is also in the running, the sources said.
Muslet and Ghalioun, however, are members of the coalition, whose rules state that only non-members can join the provisional government. A previous attempt to amend these rules failed when some coalition members keen to preserve the principle of separation of powers, threatened to resign, the sources said.
Qadi, the economist, may become front-runner for prime minister if the Syrian National Council and its allies fail to find the two-thirds majority needed to change the coalition's rules to allow Muslet and Ghalioun to run.
It is not a foregone conclusion that the often-postponed project for a provisional government, which has received only lukewarm international support, will go ahead in Istanbul.
Another idea is to form an executive body to help administer rebel-held areas without calling it a government as such.
"International powers have given political recognition to the coalition, which is an assembly, but they are not keen to recognise a provisional government because it won't have control over the rebel brigades," one coalition source said.
The coalition, formed with Arab and Western backing in Qatar at the end of last year, is broadly comprised of three voting blocs, insiders say.
The Muslim Brotherhood and its Syrian National Council and other allies can garner around 33 votes, short of the simple majority needed to elect their candidate as prime minister.
Another bloc of about 28 members revolves around the coalition's secretary-general, Mustafa Sabbagh, a Syrian businessman well connected in the Gulf who has a good working relationship with Alkhatib and with his deputy, veteran opposition leader Riad Seif, who founded the coalition.
Holding the balance are about 10 independents. They include Kamal al-Labwani, a liberal physician jailed under Assad for six years after visiting the United States to seek help in freeing political prisoners in Syria, and Haitham al-Maleh, a former judge with a long record of opposition to Assad family rule. (Reporting by Khaled Yacoub Oweis, Amman newsroom; Editing by Alistair Lyon)