Obasanjo, 66, was sworn in for another four-year term by Nigeria's Chief Justice Mohammed Uwais at a colourful ceremony in the capital Abuja.
His re-election is contested by his main challenger, former military dictator Mohammadu Buhari, as well as other opposition parties who say the April 19 poll was rigged.
Nigerian and foreign monitors also criticised the election, reporting that it was marred by widespread irregularities, but most key members of the international community have recognised Obasanjo's victory.
At least 47 countries have sent representatives to congratulate the president.
Several African presidents were present at yesterday's ceremony, including South Africa's Thabo Mbeki, Ghana's John Kufuor, Senegal's Abdoulaye Wade, Zimababwe's Robert Mugabe, Ivory Coast's Laurent Gbagbo, Mozambique's Joaquim Chissano, Sierra Leone's Tejan Kabbah and Togo's Gnassingbe Eyadema.
Former colonial power Britain sent Deputy Prime Minister John Prescott, while the US sent Education Secretary Rob Paige. – Sapa-AFP.