* New Syrian opposition PM tackles al Qaeda
* Wants to convince fighters to abandon the group, affiliates
* Ahmad Tumeh seen as moderate voice, advocates tolerance
By Khaled Yacoub Oweis
ISTANBUL, Sept 15 (Reuters) - The head of Syria's new provisional government pledged on Sunday to curb the influence of al Qaeda militants he said exploited the opposition's inability to fill a vacuum left by the collapse of President Bashar al-Assad's authority in much of the country.
In his first interview since being chosen by the Syrian National Coalition this weekend to head a 13-member cabinet, moderate Islamist Ahmad Tumeh told Reuters the opposition had to confront al Qaeda ideologically by emphasizing that democracy did not contradict Islam and deny the group popularity by restoring public services in rebel-held areas.
"On top of the destruction and killing and displacement the regime has brought, people are now suffering from militants' behaviour," Tumeh said. "The people pursued a basic quest for freedom (rather than) deeper despotism."
Tumeh, a former political prisoner who advocated tolerance in a long political career, is the most senior opposition figure to take on al Qaeda publicly.
But he faces the daunting task of establishing a central administration in rebel-held areas, where hundreds of brigades without a unified command threaten chaos and disciplined al Qaeda fighters have a significant presence.
It also remains unclear whether Tumeh can boost the opposition's credibility and relevance as high-stakes diplomacy plays out between Washington and Moscow to resolve Syria's two-and-a-half year civil war.
He does have the support of Ahmad Jarba, the coalition's president backed by Saudi Arabia, and became prime minister after a deal involving Qatar's pointman in the opposition.
The Sunni Muslim states have been the rebels' main backers, as they see defeating Assad as key to their confrontation with the Syrian leader's main regional patron, Shi'ite Iran.
CRITICISING AL QAEDA TABOO
Criticising al Qaeda, which rejects democracy and wants to establish a form of Islamic Caliphate, has been taboo among most Islamists in the Syrian opposition because of the group's role in fighting Assad's forces in the north and east of the country.
The eastern provincial capital of Raqqa partly fell under the control of groups affiliated to al Qaeda earlier this year, along with several towns in the neighbouring province of Aleppo and in Idlib.
But there have been demonstrations against the group in Raqqa after militants abducted liberal activists in the city.
Clashes broke out last week between a faction linked to al Qaeda and more moderate Islamists and Free Syrian Army fighters in Tumeh's Deir al-Zor province bordering Iraq, according to opposition sources.
In a sign of a widening rift between militants and less hardline opponents of Assad, al Qaeda leader Ayman al-Zawahri told Islamist militants in Syria to avoid alliances with other rebel fighters supported by Gulf Arab states and the West, which also back the Syrian National Coalition.
Tumeh, 48, is the secretary general of the Damascus Declaration, a group of veteran opposition figures who led a peaceful resistance to Assad before the revolt.
He was jailed several times during the uprising and was forced to flee the country earlier this year.
A dentist by profession, Tumeh was imprisoned from 2007 to 2010 along with 11 prominent opposition members who had demanded that Assad embark on democratic change in the country, which has been ruled by his family since 1970.
Tumeh said the opposition faced the "ideological challenge" of convincing many who joined al Qaeda to leave the group.
"If they refuse, then we will look for all ways to guarantee the security of the people and their livelihood and a dignified living," Tumeh said.
He said many of the ranks of al Qaeda affiliates in Syria did not have a deep attachment to the group, and joined because it provided them with weapons to fight Assad's forces and helped local communities with bread and basic staples.
"They became susceptible to the idea that religion cannot reign unless there is a religious state. This is not what Islam says," Tumeh said.
Tumeh will start talks in the next few days to name his ministers, a process expected to take weeks. The government then plans to move into northern Syria, despite the risk of air strikes and shelling by Assad's forces.
"So many have been killed for the cause of freedom. I and all the ministers (must) share the risk," he said.
The first step necessary to asserting the provisional government's authority was to take control of the border crossing with Turkey, now controlled by myriad rebel groups.
"First it has to be the crossings to secure the supply lines," he said. "Then step-by-step we will make progress to restore security to the liberated areas and restart basic services and health and education."
Tumeh has worked closely with liberals and Islamists alike, including Riad al-Turk, the main political figure in the Damascus Declaration, who at 82 still operates underground in Syria despite spending 25 years as a political prisoner. (Editing by Mike Collett-White)