He said: "The counting process should stop immediately and if that continues, it will have no legitimacy.
Mr 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.
His rival, ex-World Bank economist Ashraf Ghani, had earlier urged all parties to respect the 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, Mr Abdullah said preliminary figures and other evidence collected by his team show mass fraud.
He added: "From now on, we have no confidence or trust in the election bodies. 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 struggle for power along ethnic lines, threatening to derail attempts to transfer power democratically for the first time in Afghanistan's history.