Pressure is mounting on France to address the detention of Gabonese opposition figure and former Prime Minister Alain-Claude Bilie-By-Nze as President Brice Oligui Nguema begins a three-day state visit to Paris.

Bilie-By-Nze has been detained since April on charges of fraud and breach of trust dating back to 2008. He denies all allegations. His legal team argues the case is politically motivated and has urged French authorities to press Gabon to uphold the rule of law.

Lawyer Arthur Vercken called on Paris to use its diplomatic influence, saying Bilie-By-Nze is being held in degrading conditions. According to Vercken, the former prime minister is confined around the clock in a five-square-metre cell and allowed outside only for a few minutes each week. He said the conditions appear designed to break his client and appealed for France to defend the principles Gabon's leadership says it respects.

Vercken rejected suggestions that such intervention would amount to neo-colonial interference, arguing France has raised human rights concerns with countries beyond its former colonies.

Gabonese authorities maintain the judiciary is acting independently and deny any political involvement in the case.