President John Dramani Mahama, who narrowly defeated his rival Nana Akufo-Addo in a December 7 vote, pledged to focus on development and unite Ghanaians.
"There is a tremendous amount of work that still needs to be done. More jobs must be created. More roads must be built," Mahama, dressed all in white, told cheering crowds in the capital Accra on Monday.
That rise has not been reflected in average incomes in the cocoa and gold exporter.
The NPP said it was pushing ahead with a legal challenge to the election results.
"We have filed an action at the Supreme Court seeking to overturn the declaration of results so we don't want to be part of any event which is based on those results," said Nana Akomea, the party's director of communication.
International observers said the election was free and fair despite delays and technical problems that forced voting into a second day.
Mahama won the ballot with 50.7% of the vote to Akufo-Addo's 47.7% – just enough to avoid a second round.
Former-president John Kufuor, an NPP member, did attend the ceremony, defying the boycott and a group of young party members who gathered outside his house to persuade him not to go.
Earlier, a new parliament dominated by the ruling party took office and elected its speaker and deputies. – Reuters