A Harvard-trained economist was poised to become Africa's first elected female head of state Thursday as she built a nearly insurmountable lead in Liberia's runoff presidential election.

With about 90 percent of the vote tallied, Ellen Johnson-Sirleaf led George Weah, a former soccer star, 59 percent to 41 percent. The election is Liberia's first since the end of a 14-year civil war.

Weah, the pre-election favorite, has challenged the results, alleging that ballot boxes were stuffed. Election officials will conduct an investigation, but international observers said Tuesday's election appeared fair, and they played down Weah's charges.

Johnson-Sirleaf, who won 20 percent of the vote in the first round last month compared with Weah's 28 percent, has now won majorities in 10 of 15 counties, including two that voted for Weah the first time.

"We are dumbfounded as to how she could be leading in areas that were traditionally our strongholds," said Sam Steve Quoah, Weah's campaign spokesman.

The fraud charges marred the climax of a largely free and peaceful election that represented a critical point in Liberia's transition from civil war, which left it among the world's poorest countries, with 85 percent unemployment.

A national hero with a thin education and no political experience, Weah captured the support of young voters, including thousands of unemployed young men who once wielded Kalashnikov assault rifles as child soldiers.

But voters in the runoff appeared to favor Johnson-Sirleaf's robust resume -- she served as the country's finance minister and worked at the United Nations and the World Bank -- and her reputation for toughness.

Johnson-Sirleaf struggled to explain her past support for Charles Taylor, who presided over the country's decline in the 1990s and is now in exile in Nigeria. But she won over voters with her technocratic savvy and pledges to trim bureaucracy, promote investment and prioritize basic services.

Throughout the election campaign, under the watch of more than 15,000 U.N. peacekeepers, Liberians voiced a desire to end a three-decade cycle of violence.

Ellen Johnson Sirleaf's reputation for toughness apparently won over voters in Liberia's presidential runoff election.