Newly inaugurated Democratic Republic of Congo President Joseph Kabila has pledged to unify his country, even though opposition leader Etienne Tshisekedi argues he is his country's rightful leader.

Mr. Kabila took the oath Tuesday in Kinshasa, following last month's election that international observers say had many irregularities, during voting and counting. U.S. officials also have called the election “seriously flawed.”

Mr. Kabila insists there is no question about the legitimacy of his re-election, and election officials say he easily won. But Tshisekedi has rejected the results as fraudulent and declared himself president, saying he will take office Friday.

Mr. Kabila has suggested Tshisekedi use the appeals system to challenge the results.

The presidential and legislative polls were only the second free elections in Congo since it was torn apart by several years of warfare that ended in 2003.

Mr. Kabila first became president after his father's assassination in 2001, and later won office following a U.N.-sponsored vote in 2006.