DAKAR, Senegal — The International Criminal Court in The Hague said on Tuesday that it had acquitted the former president of Ivory Coast, Laurent Gbagbo, and one of his aides of crimes against humanity and a litany of other charges related to months of violence that followed the country’s presidential election in 2010.

The decision was a severe blow to the prosecution in the case of the 73-year-old Mr. Gbagbo, the first former head of state to reach trial at the court, and raises new questions about the effectiveness of the court to pursue its mandate of dealing with war crimes and genocide, among other crimes.

Prosecutors had built a largely circumstantial case against Mr. Gbagbo for charges stemming from the violence that followed his refusal to give up power after being ousted from office during a 2010 election. The events gave rise to a bloody civil war and left a nation deeply divided to this day.

In The Hague on Tuesday, the public gallery erupted in loud cheers and screams as the presiding judge read out the decision acquitting Mr. Gbagbo and his aide, Charles Blé Goudé. Outside the court, supporters of Mr. Gbagbo gathered with Champagne. In Ivory Coast’s economic capital, Abidjan, shirtless men ran through the streets toasting the former president. But victims of the civil war who had testified against him were devastated.