Military officers in oil-producing Gabon said they had seized power on Wednesday, placing President Ali Bongo under house arrest and naming a new leader after the Central African state's election body announced Bongo had won a third term.

Saying they represented the armed forces, the officers declared on television that the election results were cancelled, borders closed and state institutions dissolved, after a tense vote that was set to extend the Bongo family's more than half century in power.

Within hours, generals met to discuss who would lead the transition and agreed by unanimous vote to appoint General Brice Oligui Nguema, former head of the presidential guard, according to another televised address.

Meanwhile, from detention in his residence, Bongo appealed in a video statement to foreign allies, imploring them to speak out on his and his family's behalf. He said he did not know what was happening.