NAIROBI, Kenya (AP) -- A slow ballot count in Kenya's presidential vote raised questions Tuesday about the election process, but it was a decision involving hundreds of thousands of rejected ballots that made it appear likely the election will be decided in a runoff.
Nearly 330,000 ballots -- the number keeps rising -- have been rejected for not following election rules, raising criticism of voter education efforts.
The election commission chairman announced late Tuesday that those spoiled ballots, as they are called here, will count in the overall vote total. That makes it very difficult, given the tight race, for either top candidate to reach the 50 percent mark needed to win outright. A runoff election between the top two candidates is expected.
Kenya's 2010 constitution -- passed after 2007-08 election violence killed more than 1,000 people -- says a candidate wins the presidency if he or she has "more than half of all votes cast in the election." That clause made the decision on the definition of "cast" key.
Partial returns Tuesday showed an early lead for Deputy Prime Minister Uhuru Kenyatta, the son of Kenya's founding president and who faces charges at the International Criminal Court. That prompted the camp of candidate Prime Minster Raila Odinga to hold a news conference to tell supporters that more of their strongholds have yet to be counted from Monday's largely peaceful vote.
Returns for most of the day showed Kenyatta with 53 percent and Odinga with 42 percent. But Kenyatta's percentage would drop to just over 50 percent when rejected votes are counted in the total. The commission said percentages would be officially updated Wednesday.
More than half of the votes cast have yet to be counted, and observers said Odinga was likely to make gains.
"If Odinga's performance improves, as seems likely, and with this decision on the rejected votes, then it's inevitable there will be a runoff," said Nic Cheeseman, a lecturer in African politics at Oxford University who is an official observer of the election.
William Ruto, Kenyatta's running mate, blamed "foreign missions" for swaying the electoral commission on its ballot decision. The decision "is meant to deny us a first-round win," Ruto was quoted as saying by The Standard newspaper.
Kenya is the lynchpin of East Africa's economy and plays a vital security role in the fight against Somali militants. The U.S. Embassy in Kenya is the largest in Africa, indicating this country's importance to U.S. foreign policy.
The chairman of the election commission, Isaak Hassan, met with political parties Tuesday to talk about the rejected ballot issue, said Tabitha Mutemi, a spokeswoman for the commission.
Hassan acknowledged what he called "growing concern" over the slow pace of election results. He said the delays are due to high voter turnout, a large number of contested seats and long travel times for polling officers.
Candidates' percentages, he said, will be calculated "based on total votes cast." Of the rejected ballots, he said: "Of course they will have an impact in the overall result."
Hassan noted the law gives his commission seven days to perform its work, and he asked for patience "from the public, the political parties as well as the candidates."
Kenya's Vice President Kalonzo Musyoka, who is also Odinga's running mate, held an afternoon news conference -- the first by any of the major candidates -- to calm Odinga supporters who were forced to look at TV news reports of Kenyatta's lead all day.
"We wish to appeal for calm and call on our supporters to relax, because we are confident that when all votes are in (we) will carry the day," Musyoka said.
Musyoka said more Kenyatta strongholds were counted Tuesday, meaning Odinga's vote total would rise.
A European election observer gave credence to that claim, saying an electoral analysis done by the U.S. Embassy showed Odinga was likely to gain ground. The observer said he was not allowed to be identified by name.
A U.S. Embassy spokesman said the embassy did not have any comment.
Long lines of voters formed around the country Monday. Election officials estimate that turnout was about 70 percent of 14 million registered voters. Attacks by separatists on the coast killed 19 people, and other attacks were seen near the border with Somalia, but the vast majority of the country voted in peace.
In the coastal city of Mombasa on Tuesday, three suspected members of the secessionist group Mombasa Republican Council were charged in court for the murder of four police officers during elections.