"Based on our results, it appears that the election goes to the second round," Ahmad Yusuf Nuristani, head of the Independent Election Commission, told a press conference in Kabul.
Abdullah, a former foreign minister, secured 44.9 per cent of the vote, with his main rival Ghani on 31.5 per cent, according to the preliminary results.
The final official result is set to be announced on May 14 after a period for adjudication of hundreds of complaints over alleged fraud.
Nuristani, who announced that the second round would be held on June 7, said nearly seven million people voted in the April 5 election out of an estimated electorate of 13.5 million -- well above the 2009 turnout.
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Of those who voted, 36 per cent were female -- a figure likely to be seen an impressive sign of women's improving status in Afghanistan, a deeply conservative Muslim country.
The eventual winner will have to oversee the fight against a resilient Taliban insurgency as 51,000 US-led NATO combat troops leave Afghanistan this year, as well as strengthen an economy that relies on declining aid money.
Ghani, a former World Bank economist, has also vowed to fight on in a run-off.
Serious fraud allegations are being investigated in the vote to choose a successor to President Hamid Karzai, who has ruled Afghanistan since the Islamist Taliban regime was ousted in 2001.
The 2009 election, when Karzai retained power, was marred by fraud in a chaotic process that shook confidence in the multinational effort to develop Afghanistan and also marked a sharp decline in relations with the United States.