* Afghanistan due to announce results on Monday
* Abdullah says will only accept a clean vote
* Both candidates say no power-sharing deal
By Maria Golovnina and Mirwais Harooni
KABUL, July 6 (Reuters) - Afghan presidential candidate Abdullah Abdullah warned his rival on Sunday he would refuse to accept the outcome of the country's troubled election unless he saw firm evidence that the vote was completely free of fraud.
His warning, on the eve of preliminary election results due out on Monday, sets the stage for a possibly violent confrontation with his rival Ashraf Ghani, a standoff which has threatened to divide the fragile country along ethnic lines.
The deadlock over the June 14 second round run-off vote has already quashed hopes for a trouble-free transition of power in Afghanistan, a headache for the West as most U.S.-led forces continue to withdraw from the country this year.
"Nobody doubts that there was fraud in Afghanistan's election. There was mass and organised fraud," a confident Abdullah told a packed news conference.
"Only when clean votes are separated from invalid votes stuffed by individuals into ballot boxes then we will accept the result."
Abdullah, a former anti-Taliban resistance fighter, draws his support from the Tajik minority in northern Afghanistan while Ghani, a former World Bank economist, represents Pashtun tribes in the south and east of the country.
As their standoff deepened, Afghanistan has become abuzz with speculation about a broader rift along ethnic lines or even violence unless they agree to accept the outcome of the vote or come to a compromise power-sharing arrangement.
However Ghani, speaking earlier this weekend, ruled out the possibility of a coalition government or any other form of power-sharing with Abdullah, turning down a scenario which many saw as the safest way out of the impasse.
Abdullah confirmed there was no backroom solution: "There is no deal behind the curtains in exchange for peoples' votes."
Afghan election officials said on Sunday the result would be definitely announced on Monday, but no time has been set for the announcement. It has already been delayed once after the Independent Election Commission said it needed to recount votes from 1,930 polling stations due to fraud allegations.
In his address, Abdullah did not say what kind of evidence he would find convincing enough in order to declare the vote fraud-free and accept the outcome.
Ghani's aides say he is in the lead in the run-off by at least one million votes. Abdullah, for his part, has accused President Hamid Karzai, also an ethnic Pashtun, of playing a role in the alleged rigging in Ghani's favour.
Refusal by either Abdullah or Ghani to accept the outcome of the election would plunge the country into a dangerous crisis, with the possibility of a bloody standoff between the two ethnic groups or even secession of parts of the country.
Without a clear leader accepted by all sides, Afghanistan could split into two or more fiefdoms along tribal fault lines, or even return to the bloody civil war of the 1990s.
It will also create a hurdle for the Unites States which hopes for a swift transfer of power in order to sign a security pact allowing some U.S. forces to stay in the country. Both Abdullah and Ghani say they would sign it promptly. (Writing by Maria Golovnina; Editing by Stephen Powell)