Liberia's President Ellen Sirleaf Johnson, Africa's first democratically elected female leader, has easily won a second term in a run-off boycotted by her rival.

With nearly all precincts reporting , the election commission said Thursday that Mrs. Sirleaf swept more than 90 percent of the vote in Tuesday's poll.

Earlier Thursday opposition candidate William Tubman said he would not accept the results.

The leader of the opposition Congress for Democratic Change boycotted the run-off after claiming it was rigged in favor of President Sirleaf.

Election observers from the Carter Center said Thursday the run-off was credible and met international standards for democratic elections.

The U.S.-based monitoring group said democracy in Liberia still faces “important challenges.” But the Carter Center dismissed as “unsubstantiated” claims by Mr. Tubman that the election process was seriously flawed.

Speaking to reporters on Thursday, Mrs. Sirleaf insisted the electoral process was legitimate and vowed to reach out to CDC and other party leaders.

Analysts say the legitimacy of her re-election may be marred by the boycott, low voter turnout and deadly clashes between police and opposition protesters just before the run-off. The Liberian government has also been criticized for responding to the unrest by closing three pro-opposition radio stations.

The Carter Center said the events suggest a “significant failure” by the government to legally resolve electoral disputes.

The monitoring group urged Mrs. Sirleaf to move quickly to appoint an independent panel to investigate the use of deadly force against protesters.

President Sirleaf on Thursday said the protesters were violating the law because they were rallying less than 24 hours before the poll. She promised an investigation into the shootings.