LIBREVILLE, Aug 30 (Reuters) - A group of senior Gabonese military officers appeared on national television in the early hours of Wednesday and said they had taken power, after the state election body announced President Ali Bongo had won a third term.

Appearing on television channel Gabon 24, the officers said they represented all security and defence forces in the Central African nation. They said the election results were cancelled, all borders closed until further notice and state institutions dissolved.

Loud sounds of gunfire could be heard in the capital Libreville, a Reuters reporter said, after the television appearance.

There was no immediate comment from the government of the OPEC-member nation.

"In the name of the Gabonese people ... we have decided to defend the peace by putting an end to the current regime," the officers said.

Tensions were running high amid fears of unrest after Saturday's presidential, parliamentary, and legislative vote, which saw Bongo seeking to extend his family's 56-year grip on power while the opposition pushed for change in the oil and cocoa-rich but poverty-stricken nation.

A lack of international observers, the suspension of some foreign broadcasts, and the authorities' decision to cut internet service and impose a night-time curfew nationwide after the poll had raised concerns about the transparency of the electoral process.