Abdullah Abdullah made the call today during a news conference.
Abdullah's team has questioned what it has determined is a 1 million vote lead by Ashraf Ghani Ahmadzai in the early tallies.
Abdullah pointed out it is a dramatic increase from the first round that put him in the lead with 45 per cent of the vote compared to 31.6 per cent for his rival.
An initial turnout estimate suggested 7 million vote in Saturday's vote, which would be 60 per cent of the 12 million eligible voters and equivalent to the first round of voting on April 5.