Guy Nzouba-Ndama, the head of the opposition in Gabon, was detained at the Congolese border five days ago with suitcases containing about $2 million (£1.8 million).
He has since been placed under house arrest.
The opening of the baggage during his detention was captured on camera and shared online.
Mr. Nzouba-Ndama, who may run for president in the 2018 elections, has been accused of breaking import laws and collaborating with foreign governments.
The accusations, according to his party, the Democrats, are made for political reasons.
The 76-year-old served as speaker of the National Assembly for 19 years and was an ardent supporter of the late president Omar Bongo, the father of the current president Ali Bongo Ondimba.