"We have obtained a vast body of evidence against the two accused," Bensouda told the Hague-based court as the trial of Gbagbo and former militia leader Charles Ble Goude got underway five years after 3,000 people died in political upheavals following the bitterly disputed 2010 elections.
Gbagbo, 70, and 44-year-old Ble Goude earlier pleaded not guilty to four charges of crimes against humanity including murder, rape, and persecution.
"The Ivory Coast descended into chaos and was the theatre of unspeakable violence," some of which was orchestrated by Gbagbo, Bensouda said.
The former strongman had "intended to stay in power by all means."
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"Nothing would be allowed to defeat Mr Gbagbo, and if politics failed, violence was seen as politics by other means."
Armed forces, mercenaries and youth groups transformed into militias were "used to attack civilians," Bensouda said, adding that Gbagbo had exercised control through "formal and parallel chains of command."
Despite being "in a position to do so," Bensouda maintained Gbagbo "never ordered the violence to stop."
Bensouda also reacted to criticism that she was only prosecuting crimes committed by Gbagbo's camp during the violence.
"We are investigating both sides of the conflict. It takes time and I encourage the people of Cote d'Ivoire to be patient," she said.