'The initial reports we are receiving are alarming,' Abdullah Abdullah told a news conference three days after the election.
'There might have been thousands of violations throughout the country, no doubt about it,' he said.
Counting is under way following Thursday's vote, but official preliminary results are not expected until at least Tuesday.
President Karzai was expected to win the ballot, according to polls.
However, his margin of victory may not be enough to prevent a second-round run off against Mr Abdullah.
