The ruling is a blow to prosecutors who have lost other major cases in recent years.

Presiding Judge Cuno Tarfusser said that prosecutors failed to prove their case and Gbagbo and co-defendant Charles Ble Goude, a close ally and former political youth leader, should be set free.

"The majority finds that the prosecution has failed to demonstrate there was a common plan to keep Mr. Gbagbo in power that included the commission of crimes against civilians," Tarfusser said.

Outside the courthouse, dozens of Gbagbo supporters, many who travelled to The Hague by bus from Paris, broke into cheers and dancing when the verdict was announced.

"Ooh-la-la, the judge completely dropped the charges," said Gbagbo supporter Olivier Kipre in the Ivorian capital Abidjan.

"I'm so joyful. I will become crazy today because I didn't believe he would be released."

Gbagbo was the first former head of state to stand trial at the ICC.

Acquittal was a major setback for the prosecution, stung by defeats in cases against Jean-Pierre Bemba, the Congolese ex-vice president released last year after his war crimes conviction was overturned, and Kenyan President Uhuru Kenyatta, who had charges against him dropped in 2015.

Prosecutors have only won three war crimes convictions over the past 15 years.

Gbagbo, 73, and Goude, had been on trial since 2016 for war crimes allegedly committed under Gbagbo's leadership.

The former leader faced four counts of crimes against humanity, including murder, rape, persecution and other inhumane acts during post-electoral violence in Ivory Coast between December 2010 and April 2011, when Gbagbo refused to accept defeat by rival Alassane Ouattara.

"There is no need for the defence to submit further evidence as the prosecutor has not satisfied the burden of proof," added the judge Tarfusser.

Gbagbo heard the ruling dressed in a dark blue suit, and the judge had to order supporters in the public gallery to calm down as they erupted in cheers.

Australian Associated Press