Gbagbo has been living in Brussels pending Wednesday's decision but plans to go home if it goes his way, thanks to an olive branch offered by his erstwhile rival.
Gambian-born Bensouda said the formal investigation followed a "painstaking" five-year preliminary probe, and vowed it would be conducted "independently, impartially and objectively, without fear or favour."
"A total of 15 of my family members were killed during the attack and very many people were injured," said farmer Muhammed Olanya, 38, adding he became head of his family "at a very young age."