One tribal elder has admitted to the BBC that he tampered with hundreds of ballots in favour of incumbent President Hamid Karzai.
 

More than 600 serious complaints are being investigated, but the deadline for new complaints has now passed.

With 60% of polling stations having already declared, Mr Karzai has a clear lead.
 

In the latest case of alleged fraud uncovered by the BBC, a tribal elder from Zaziaryoub district - in the eastern province of Paktia - said he had helped to fill in about 900 ballots in favour of President Karzai.
 

The elder says in a neighbouring village, his nephew saw one man fill in more than 2,000 ballots.
 

Allegations of fraud have been made against all the prime candidates, but the election process seems to have been working overwhelmingly in favour of Mr Karzai, says the BBC's Chris Morris in Kabul.
 

However, some of these complaints will not get heard by the Electoral Complaints Commission, as the time to file an official complaint has passed.
 

The commission is currently looking into 2,000 fraud claims overall.
 

Figures obtained from the campaign of Hamid Karzai's leading opponent, Dr Abdullah, suggest that in four provinces alone results have been declared from 28 polling stations which observers had reported were closed.

 

Damning evidence

AFGHAN ELECTION Vote held on 20 August for presidency and provincial councils Turnout not made official yet but estimated at 40-50% More than 400 insurgent attacks on polling day, Nato says More than 2,000 fraud allegations, 600 deemed serious Final result expected 17 Sept but fraud allegations must be cleared Hamid Karzai has clear lead over Abdullah Abdullah in presidency race Candidate needs more than 50% to avoid runoff

Just days ago, a tribe in the south made the most serious claim so far.
 

The leader of Kandahar's Bareez tribe said that nearly 30,000 votes were cast fraudulently for President Hamid Karzai instead of primarily for the main challenger, Abdullah Abdullah.
 

Mr Karzai's brother, Ahmed Wali Karzai, who heads the Kandahar provincial council, called the claims "baseless".
 

Because of time needed to investigate the fraud allegations, the final results of the election may not be known until the end of September.
 

There are concerns continuing claims of fraud could undermine the legitimacy of the election, which Afghanistan's Western allies see as crucial in their campaign against the Taliban.