Abdullah Abdullah: 'There is no doubt that the state crafted an ingenious fraud'
Initial counting from Afghanistan's presidential election shows incumbent Hamid Karzai with a slight lead.
With 10% of the ballots counted, the election commission said Mr Karzai had 212,927 votes, compared to 202,889 for ex-Foreign Minister Abdullah Abdullah.
Final results are not expected for several weeks. A candidate needs more than 50% of votes to avoid a run-off.
Soon after the figures were announced, an explosion struck the southern city of Kandahar, killing dozens.
Earlier it was reported that four US soldiers had been killed by a bomb in southern Afghanistan, bringing the number of foreign forces killed in 2009 to 295, the deadliest year since the US-led invasion to overthrow the Taliban.
'Too early to call'
The Independent Election Commission said that so far 524,444 valid votes had been counted, with Mr Karzai on 40.6% and Mr Abdullah on 38.7%.
Martin Patience BBC News, Kabul
The results have been compiled from polling stations across the country, but are not necessarily indicative of the national picture.
They do seem to be suggesting a closer race than expected, but people will be asking where these votes came from. Many seem to be from the north, where Mr Abdullah would expect to pick up ballots.
A spokesman for the election commission was at pains to stress that this was only a partial result - 10% of the total vote. These figures also suggest that voter turnout was about 35% - less than half that of the first presidential race in 2004.
There have been widespread allegations of fraud and Western officials say that for this vote to be credible these must be thoroughly investigated.
Ramazan Bashardost has 53,740 votes so far and Ashraf Ghani Ahmadzai is fourth on 15,143.
Only 2% of votes in Kandahar province have been counted and none in Helmand. Mr Karzai is expected to do well in both southern provinces.
The commission says it will release more results over the next few days.
Before the announcement of the first results, Mr Abdullah called on Afghans to react calmly.
"I'm urging Afghans... to be patient and to show responsibility. I think that the people don't want to resort to violence," he said.
Washington's special envoy to Afghanistan and Pakistan, Richard Holbrooke, said on Tuesday the initial results were not conclusive.
"You don't call it with 10%... it's too early to call," he said.
The BBC's Ian Pannell in Kabul says any preliminary claims about the result must be viewed with caution in the light of the allegations of fraud, corruption and ballot-stuffing and concerns about low voter turnout, especially in the south.
Other leading candidates:
Ramazan Bashardost 53,740 (10.2%)
Ashraf Ghani Ahmadzai 15,143 (2.9%)
Winning candidate needs more than 50% of votes to avoid a run-off
The election commission is also being urged to wait until the official adjudicators, the Electoral Complaints Commission (ECC), completes its investigation.
There are almost 800 complaints of fraud and irregularities and, out of these, 54 are considered to be very serious.
Mr Abdullah has said that he has evidence that voting was widely rigged in favour of Mr Karzai. Mr Abdullah said he had submitted the allegations to the ECC.
Another leading presidential candidate, Mirwais Yasini, told the BBC that workers from his campaign discovered about 800 ballots with ticks next to his name that he believes had been discarded from the ballot box.
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