International Criminal Court judges made "fundamental and serious" errors when they cleared former Ivory Coast president Laurent Gbagbo of crimes against humanity last year, prosecutors said as they launched an appeal on Monday.
Gbagbo and his right-hand man Charles Ble Goude were acquitted in January 2019 of charges over post-electoral violence in the restive West African nation in 2010-11 in which around 3,000 people died.
Prosecutors want the acquittal overturned and a retrial at the court in The Hague, which was set up in 2002 to deal with the world's worst crimes.
Helen Brady, senior appeals counsel for the ICC prosecution, said the gravest mistake was that judges did not make a proper written judgment at the time, instead handing down the decision to acquit the Ivorian pair orally.
Brady said this was "no harmless procedural irregularity".
"It tarnished the very essence of the judicial adjudicative process, thereby affecting the reliability and the integrity of the decision itself," she said.
The prosecution said it had presented key elements of proof with 4,610 documents and 96 witnesses interviewed during the trial.
Gbagbo, now 75, appeared by videolink for the hearing, which was conducted remotely because of coronavirus measures. He appeared bearded and wearing a casual top and listened through headphones.
The one-time Ivorian strongman, the first former head of state to be tried by the ICC, was last month allowed to leave Belgium where he had been hosted under strict conditions since his release last year.
Gbagbo had previously spent eight years behind bars in The Hague before his surprise acquittal by the court.
The ICC said in a statement it "will make its judgment on this appeal at a later stage."
'Justice was not served'
Chief ICC prosecutor Fatou Bensouda formally appealed the trial judges' decision in October.
She said the judges did not issue their written verdict until some six months after the oral acquittal.
Judges had also cleared the pair "without properly articulating and consistently applying a clearly defined standard of proof", Bensouda added.
Gbagbo's Ivorian Popular Front (FPI) has called on Ivory Coast's President Alassane Ouattara to engage in a "dialogue" over his return to the country.
But an association for victims of the violence expressed "energetic opposition" against Gbagbo coming home as the country braces for tense elections scheduled for October.
Gbagbo technically faces being jailed on his return after being sentenced in absentia to a 20-year term by an Ivorian court last November for the "looting" of the local branch of the Central Bank of the West African States (BCEAO) during the post-election crisis.
Monday's hearing comes as the ICC prosecutor's office fights a string of acquittals and failed cases including that of Congolese politician Jean-Pierre Bemba, acquitted in 2018 of war crimes and crimes against humanity.
It also comes at a time when the ICC is under assault from the administration of US President Donald Trump because of the tribunal's probe into crimes committed by US forces during the war in Afghanistan.