President Omar Bongo of Gabon has been sworn in for a further seven-year term.
Omar Bongo has been in power since 1967
Mr Bongo, Africa's longest-serving head of state, won an election in November, 38 years after he first came to power.
Opposition candidates say the poll was rigged and that Mr Bongo had used the country's oil wealth to buy votes.
About 20 African heads of state were expected to be in Gabon for the ceremony, including the presidents of South Africa, Nigeria and Senegal.
Officials have described the ceremony as a mini-summit to prepare for next week's African Union summit in Sudan, where a new AU president is expected to be named.
Journalist Andrew Manley says the president's length of service has given him good connections, making him a respected power broker in Africa.
But he says not all people in oil-rich Gabon have benefited from Mr Bongo's time at the top.
"According to census data and UN and World Bank studies, the disparity in wealth in Gabon is probably per capita the highest in the world and the Bongo era has done very very little in this respect," Mr Manley told the BBC's Focus on Africa programme.
"A great deal of Gabon's accumulated wealth during the Bongo era has been exported," he said.
Results from November's election gave Mr Bongo 79.2% of the vote.
His two main opposition rivals, Pierre Mamboundou and Zacharie Myboto, said the results were not valid, and hundreds took to the streets of the capital, Libreville, in early December to protest at the results.
Last year's election was the third he has won since a multi-party system was introduced in 1991.