A former agriculture minister, an economist and a communications exective lead the race in a vote that could change the course of Syria's conflict.
The rebel premier's first task would be to appoint a new government, which would be based inside Syria.
While it would boost the opposition's credibility, a rebel government would have little chance of dialogue with the regime of President Bashar al-Assad.
The umbrella Syrian National Coalition cautioned there was no guarantee that the election wpi;d take place as scheduled, with the process postponed before.
In rebel-held parts of Syria, opinion is divided between residents desperate for basic services and the rule of law, and those who feel the coalition is ill-suited to choose a competent administration.
"The coalition is not close enough to the ground to have a real sense of the needs here," Aleppo-based activist Abu Hisham said.
Matar Ismail, an activist in Damascus, disagreed, saying there was "a real need in the liberated areas for better administration of daily life".
"There are more than 10-million Syrians in liberated territories who need education and health services," coalition spokesperson Walid al-Bunni told said.
"But should there not be an election tomorrow [Monday], there will be a need for more discussion with [rebel] local councils and the Free Syrian Army groups fighting in Syria," he added.
Opposition sources said former agriculture minister Asaad Mustapha, economist Osama Kadi, and communications executive Ghassan Hitto were frontrunners for the vote.
Should they reach a consensus, those gathering in Istanbul were likely to pick a good administrator with long-standing ties to the uprising, although nations backing the rebels, including Qatar and Saudi Arabia, were likely to influence the choice.
"The prime minister must be a man who is completely with the revolution, and it is better that it be someone who was in Syria until recently, not someone who has lived abroad for a long time," opposition figure Haytham al-Maleh said.
"There's a push to ensure the interim prime minister is a technocrat," another coalition member said, speaking on condition of anonymity.
Aleppo-born Kadi, founder of the Syrian Centre for Political and Strategic Studies in Washington, is favoured for his technocratic background, as is Hitto, who has lived in the United States for decades.
Mustapha brings eight years of experience as a minister under Syria's former president Hafez al-Assad.
"If what's wanted is a technocrat then perhaps Osama Kadi will win. And if the choice is based on who has experience and is the most capable politically, it will be Asaad Mustapha," coalition member Ahmed Ramadan said.
"The latter has good experience … and he has been close to the revolution from its beginning and is respected."
Several prominent opposition figures are not in the running, including Christian dissident Michel Kilo, who has said he will not stand.
The coalition is expected to hold an initial vote, followed by a run-off between the top two candidates. The winner will then choose a Cabinet, which must be approved by the coalition.
The United States was believed to oppose the creation of an interim government, fearing it could hamper efforts to start a dialogue with the regime but the process was backed by Turkey and much of the Arab League. – AFP