Afghan presidential candidate Abdullah Abdullah said on Wednesday that he had suspended work with the country's electoral bodies and asked them to stop counting votes because of widespread fraud.
"The counting process should stop immediately and if that continues, it will have no legitimacy," Abdullah told reporters.
Abdullah's refusal to work with the country's election commissions has pushed the country into a deeper political crisis just days after the run-off vote.
Abdullah's rival, ex-World Bank economist Ashraf Ghani, was not immediately available to comment but earlier urged all parties to respect the political process.
Afghanistan's foreign backers have long feared a worst case scenario in which a close outcome coupled with widespread fraud would give the losing candidate ground to refuse to accept defeat.
While ballot boxes have yet to reach the capital Kabul for counting, Abdullah says preliminary figures and other evidence collected by his team show mass fraud.
"From now on, today, we announce that we have no confidence or trust in the election bodies," Abdullah continued. "I call on all our observers to abandon monitoring and return to our provincial offices."
Observers fear allegations of fraud on both sides could lead to a lengthy and paralysing struggle for power along ethnic lines, threatening to derail attempts to transfer power democratically for the first time in Afghanistan's history.
Based on initial reports received by Abdullah's staff, results from Saturday's vote put Ghani in the lead by close to a million votes.