Dbeibah's entry into a race that now features many of Libya's main players of the past decade of chaos adds to the turmoil over a vote that is due to take place within five weeks, but for which rules have not yet been agreed.

Parliamentary and presidential elections on Dec. 24 were demanded by a U.N. political forum last year as part of a roadmap to end Libya's civil war, a process that also led to the formation of Dbeibah's interim unity government.

Libya has had little stability since the 2011 NATO-backed uprising that ousted Muammar Gaddafi as the country fragmented among myriad armed groups. Government was split in 2014 between warring rival administrations based in east and west.

However, the disputes over the election threaten to derail the U.N.-backed peace process that emerged last year after the collapse of an eastern military offensive to seize the capital Tripoli.

The elections are being organised under a law issued by parliament speaker Aguila Saleh in September that set a first-round presidential vote for Dec. 24 but that delayed the parliamentary election to January or February.

Dbeibah and some major political figures and groupings in western Libya have criticised Saleh's election law, saying it was passed improperly, and have called for both votes to be delayed until there is agreement on the rules.

The electoral commission and Libyan courts are likely to rule on the eligibility of candidates in the coming weeks - a process that may itself stir new disputes.

DISPUTES

Dbeibah is likely to be a frontrunner in the election after implementing a series of populist spending measures in recent months including infrastructure projects and payments to support young newlyweds.

The 63-year old hails from one of Libya's wealthiest business families, but he was not a prominent figure in his own right before the U.N. political forum chose him to lead the interim government overseeing the run-up to elections.

As prime minister, he has pledged investment in Libyan regions that suffered neglect during the past decade of chaos, agreed major contracts with countries involved on both sides of the civil war and courted young people with financial support.

He has not yet said publicly why he has chosen to break the televised promise he made when he was appointed that he would play no role in the election.

Saleh's law might also rule him out as a candidate because it requires him to step down from his position three months before the vote, which he did not do.

His best-known rivals include Gaddafi's son and one-time heir apparent Saif al-Islam Gaddafi, the commander of the eastern forces in the civil war Khalifa Haftar, and Saleh himself.

Gaddafi and Haftar are both accused of war crimes, which they deny, and would be seen in swathes of Libya as unacceptable after years of warfare.

Reporting by Reuters Libya newsroom, writing by Angus McDowall, editing by Andrew Heavens and Hugh Lawson