Early figures from campaign team observers suggest Karzai has won 72 per cent of the vote with his closest rival Abdullah Abdullah gaining 23 per cent, The Telegraph reports.
If confirmed, the scale of the win will provoke accusations of vote-rigging with electoral officials saying that they are already investigating dozens of complaints of fraud on a scale profound enough to sway the result.
The first provisional results are not expected until Tuesday, Aug 25 with final results following weeks later after complaint rulings.
Karzai needs over the 50 per cent of the vote to avoid a second round run off against his former foreign minister.
However, it will be hotly contested after the Electoral Complaints Commission (ECC) said it had received 225 complaints since polls opened on Thursday for the presidential and provincial council elections.
"Thirty five have been assigned a high priority and these are ones that we had to deem to be material to the outcome of the election results," said Grant Kippen, head of the commission.
Dr Abdullah said his campaign team had received alarming reports of irregularity.
"There might have been thousands of violations throughout the country, no doubt about it," he said.
His allegations were dismissed by a spokesman for the Karzai campaign, who said it had also filed complaints about Dr Abdullah's supporters' activities.
Waheed Omer said unsuccessful campaigns would file complaints to "try to justify their loss".
The Free and Fair Election Foundation of Afghanistan claims to have recorded instances of multiple and underage voting.