ATHENS (Reuters) - Greek Prime Minister Alexis Tsipras's divided leftist Syriza party met on Thursday to thrash out its differences amid a mounting rebellion by far-left lawmakers who accuse the party of betraying its anti-austerity roots.

Greece narrowly averted an exit from the euro zone for now when it struck an 11th-hour deal with lenders this month on a third financial rescue, but that has cost Tsipras the support of about a quarter of his lawmakers and put Syriza on the brink of a split.

The deepening crisis within Syriza is the most serious political challenge to Tsipras, who otherwise enjoys unrivalled domination of the Greek political scene and remains popular despite his sudden U-turn to accept stringent bailout terms.

Failure to assert his grip on Syriza and quash the far-left revolt could plunge Greece back into turmoil and risk derailing talks with European and International Monetary Fund lenders on a new 86 billion euro aid package to keep Athens afloat.

Roughly 200 members of Syriza's decision-making central committee gathered at an old movie theater in central Athens to decide whether to hold a regular, emergency congress or a referendum to redefine the party's strategy.

"At the moment there are two different strategies competing in the same party - one that wants Greece inside the euro and the other that wants Greece outside euro," Olga Gerovasili, the government spokeswoman told reporters as she arrived.

"These two can't exist together at the government level."

The hardline Left Platform faction is demanding that Syriza abandon talks with lenders immediately and hold an ordinary party congress to determine the party's course. A regular congress would favor the far-left, with the same members who attended two years ago returning to vote on Syriza's future.

Tsipras is believed to favor an emergency congress instead, which would enable him to bring in new Syriza members and capitalize on the wider public support he has secured over the past two years, making it easier to defeat the far-left camp.

A less likely course would be for the central committee to opt for a referendum, which would put an as yet undetermined question about party strategy to grassroots members for a vote.

In a radio interview on Wednesday, Tsipras said he could be forced to call an early general election if he no longer had a parliamentary majority, and suggested an emergency party congress could be held in early September.

That would potentially allow Greece to wrap up negotiations with lenders on a bailout package, which Athens hopes to have sealed before a major debt payment falls due on August 20, and secure initial funding before a vote.

"Our priority is the (bailout) deal," Gerovasili said. "After this we can deal with party issues."

Negotiations with officials of the European Commission, the European Central Bank, the IMF and the euro zone's rescue fund, the European Stability Mechanism, were continuing discreetly in Athens on Thursday.