Lawyers representing Gabon opposition leader Jean Ping sent a case file to the International Criminal Courts prosecutor on Thursday, accusing the government of crimes against humanity.
The accusation relates to violence which broke out for two days following the controversial reelection of President Ali Bongo in August, a result questioned by the European Union.
The opposition claimed 26 people were killed during the riots and protests that begun on August 31, although official government figures put that toll at just three.
The file sent to the ICC was "the fruit of three months of investigations carried out in Gabon and abroad, which demonstrate the existence of crimes against humanity committed by the Gabonese authorities", French lawyer Emmanuel Altit said in a statement.
"It transires that Gabonese government forces launched, in particular on August 31 in Libreville, a planned attack on the civil population aimed at maintaining power" for Bongo, the statement said.
Back in September, the ICC's chief prosecutor Fatou Bensouda opened an initial probe into the deadly violence, but at Libreville's behest.
Opposition protestors had set parliament ablaze and clashed with police.
Ping has styled himself as president-elect since losing the August election by less than 6,000 votes.
That result lacked "integrity", according to EU observers who cited "anomalies", including a flagrant one in which Bongo's Haut-Ogooue heartland returned a 99 percent turnout -- with the president taking 95 percent of the vote -- compared to a 54.24 percent turnout across the rest of the country.
The ICC prosecutor must now study the case before determining whether or not to open an inquiry.
Gabon is a signatory to the Rome Statute which provides the guidelines for prosecutions at the ICC, set up in 2002 in The Hague to try the world's worst crimes.