text: Opposition Alleges Afghan Election Fraud KABUL, Afghanistan - Afghanistan's first direct presidential election was thrust into turmoil hours after it started Saturday when all 15 candidates challenging interim leader Hamid Karzai said they would boycott the outcome because of potential fraud in the system designed to keep voters from casting more than one ballot.    The poll also was marred by violence, as militants ambushed police escorting boxes of ballots, killing three officers in the deadliest direct assault on the landmark post-Taliban vote, a senior official said...