By Paul Tait and Sayed Salahuddin

KABUL (Reuters) - The main challenger to Afghan President Hamid Karzai in last week's elections said on Sunday he had evidence the ballot had been widely rigged.

With counting underway following Thursday's vote, the country is on tenterhooks ahead of an official result -- although the start of the Muslim fasting month of Ramadan and a relative lull in violence has helped calm tensions.

An election result respected by the candidates and their supporters is crucial for the country and U.S. President Barack Obama, who has made stabilizing Afghanistan his top foreign policy priority.

On Sunday former foreign minister Abdullah Abdullah, who was given a fighting chance of pushing the election to a second round, said he had evidence of widespread rigging.

"The initial reports we are receiving are alarming," he said. "There might have been thousands of violations throughout the country, no doubt about it."

In a separate news briefing, the country's election watchdog said it was dealing with scores of complaints, but there was no sign they would directly affect the result.

Two opinion polls ahead of the election predicted Karzai would win, but not by enough to prevent a second round run-off against Abdullah.

The Election Complaints Commission (ECC) said it received 225 complaints of which 35 had been labeled a priority.

"The allegations contained in the complaints we have received so far range from voter intimidation, violence, ballot box tampering (to) interference by some IEC (Independent Election Commission) officials," Grant Kippen told a news conference.

Kippen said the ECC was aware of "significant complaints" of vote irregularities, but that there were no specific charges against individual candidates such as Karzai.

Millions of Afghans braved threats of Taliban violence to vote in what was only the country's second presidential election.

With the outcome still unpublished and both sides claiming victory, Washington's envoy to the region, Richard Holbrooke, said both Karzai and Abdullah has promised to respect the result.

"So the United States' position, and that of all our NATO allies, is unanimous: we all will respect the decision of the Independent Election Commission," he said on a visit to the western province of Herat on Sunday.

Western and Afghan officials have breathed a sigh of relief that violence did not wreck the election altogether after Taliban militants vowed to disrupt it and launched sporadic attacks across the country on the morning of the poll.

Attacks and threats did scare many people away, however, especially in the Taliban's southern heartland. Since voters in the south were expected to back Karzai, poor turnout there increases the chance of a run-off.

There has been a relative lull in violence since the vote, coinciding with the Muslim fasting month of Ramadan, which began on Friday night.

The prospect of an election dispute has led to fears of unrest, especially if it takes on an ethnic or regional character in a country where competing groups have often taken up arms.

(Editing by David Fox and Jerry Norton)