The election commission has said it is investigating more than 2,500 complaints, including accusations of voter intimidation and large-scale ballot-stuffing, which it must investigate before final results can be announced.
The allegations, along with low turnout in the violent south, could strip the election of legitimacy and prove a big embarrassment to the United States and other countries that have staked their Afghan policies on support for a credible government to combat the Taliban, corruption and the country's huge drug trade.
Top envoys for Afghanistan held a meeting in Paris on Wednesday to chart a way forward amid the claims of fraud.
Richard Holbrooke, the US special representative, joined counterparts from 26 countries and organisations for talks about how to deal with ballot-stuffing charges tarnishing Karzai.
Abdullah on Wednesday accused the commission of co-operating in the alleged fraud.
In the northern city of Mazar-i-Sharif, an official with Abdullah's campaign warned that his supporters could take to the streets if there is a perception that fraud is being overlooked.
"Dr Abdullah is meeting with foreign embassies and regional partners to try to find a solution," said Zalmai Younosi, the campaign chief for six provinces.
"If there is no result, then it is protest and violence," he said.
Ahead of the meeting, Holbrooke said Washington was neutral in the campaign and that allegations of ballot fraud would be addressed.