Provisional results released on Monday - ahead of a voting fraud investigation - showed that Abdullah had garnered some 3.5 million votes or 44 percent of the total.

That left him trailing Ghani, with 4.5 million votes or 56 percent, by some distance, even if a significant number of ballots are invalidated.

"We don't accept the results which were announced today and we consider this as a coup against people's votes," said Mujib Rahman Rahimi, a spokesman for the Abdullah campaign.

The commission has already acknowledged that vote rigging had occurred, with ballots from about 7,000 more of the nearly 23,000 polling stations to be audited. Afghan officials released the preliminary results despite an earlier warning from one-time Foreign Minister Abdullah that he would not accept any results until all fraudulent ballots have first been invalidated.

Ghani supporters in the capital, Kabul, celebrated the apparent lead of their candidate by banging drums and firing guns. Some supporters of Abdullah, a former anti-Taliban fighter, were said to have driven cars around the city in protest.

'Not final or authoritative'

The announcement also brought a swift and strongly-worded statement from the US State Depatment, which cautioned that the results were "not final or authoritative." It urged electoral authorities to "implement a thorough audit whether or not the two campaigns agree."

Any disagreement about the election outcome could cause tensions across the ethnic divide. Ghani - a former finance minister and - attracts much of his support from Pashtun tribes in the south and east of the country, while Abdullah's adherents tend to include more Tajiks and other northern Afghan groups.

President Hamid Karzai, who has presided over Afghanistan since the US-led invasion of the country in 2001, is constitutionally barred from a third term in office. The country's next president will be tasked with leading the country through one of its greatest tests since the invasion, with foreign forces in the country set to leave after 13 years of combat with the Taliban.

rc/kms (AFP, AP, Reuters)