By Paul Tait and Sayed Salahuddin

KABUL (Reuters) - Afghan President Hamid Karzai's main challenger said on Sunday he had evidence last week's election had been widely rigged by the incumbent and that he had lodged more than 100 complaints.

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 for U.S. President Barack Obama, who has made stabilizing Afghanistan his top foreign policy priority.

On Sunday former foreign minister Abdullah Abdullah, whom polls gave a fighting chance of pushing the election to a second round in October, said he had evidence of widespread rigging. Abdullah and Karzai's camp both say they are ahead in the vote count.

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

Abdullah said his team had already lodged more than 100 complaints with election officials.

"Widespread rigging has taken place by the incumbent, through his campaign team, and through the state apparatus, through government officials," Abdullah said.

"This has to be prevented. That's critical for the survival of the process and that's critical as far as the hope for a better life of the Afghan people is concerned," he said.

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.

The Independent Election Commission also said partial results would be released on Tuesday, and repeated its warning to candidates that they should not make premature declarations.

PRIORITY COMPLAINTS

Two opinion polls ahead of the election predicted Karzai would win, but not by enough to prevent a second round run-off against Abdullah. Karzai must win more than 50 percent of the vote to avoid a runoff

Watchdog the Election Complaints Commission (ECC) said it had 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.

Abdullah said the southern provinces of Ghazni and Kandahar -- the birthplace of the Taliban -- were major areas of concern. He said vastly exaggerated voter turnouts were being reported, as well as ballot box stuffing well after the actual vote.

With the outcome still unpublished and both sides claiming victory, Washington's envoy to the region, Richard Holbrooke, said Karzai and Abdullah had both 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

(Additional reporting by Golnar Motevalli; Editing by David Fox)