Election observers from the Carter Center say Liberia's presidential run-off this week was credible and met international standards for democratic elections.

In a statement Thursday, the U.S.-based monitoring group said democracy in Liberia still faces “important challenges.” But the Carter Center dismissed as “unsubstantiated” claims by the opposition that the election process was seriously flawed.

Liberia's election commission is expected to announce the outcome of the November 8 run-off by the end of the week. Opposition candidate William Tubman has already said he will not accept the results.

The leader of the opposition Congress for Democratic Change boycotted Tuesday's run-off, saying election officials planned to rig it in favor of President Ellen Johnson Sirleaf. She is now poised to win a second term.

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.