Former Ivory Coast president Laurent Gbagbo pleaded not guilty on Thursday to charges that he sparked a civil war in his country that killed 3,000 people.
His co-accused, youth leader Charles Ble Goude, also pleaded not guilty, saying he did not recognise the charges.
The war began in early 2011 after Gbagbo refused to accept the result of an election he lost in late 2010. He remains a divisive figure at home and his trial in The Hague could rekindle tension in the world's largest cocoa grower.
Gbagbo, who was finally succeeded by his rival Alessane Ouattara after France intervened to put an end to four months of violence, is the most senior politician to stand trial at the court since it was set up 13 years ago.
He faces four counts of crimes against humanity and war crimes alongside his co-accused, youth leader Charles Ble Goude.
Hundreds of supporters of former Ivory Coast president Laurent Gbagbo gathered outside the International Criminal Court as the trial began.
His supporters say Gbagbo is being punished for standing up to the former colonial power.
"We want him to be released," said Michele, an Ivorian citizen living in Paris who travelled the 500 km
from the French capital to attend the trial opening.
"This is neo-colonialism. France intervened to oust Gbagbo and install a rebel chief," she said, referring to Ouattara, who many of Gbagbo's supporters regard as an illegitimate leader despite his re-election last year.
Gbagbo's supporters accuse the court of being selective in its prosecutions, though court prosecutor Fatou Bensouda said on Wednesday she had stepped up investigations into the pro-Ouattara camp.
The case is also a test for the credibility of the global war crimes court, whose previous attempt to press charges against a top politician, Kenya's President Uhuru Kenyatta, ended in disarray amid a fierce diplomatic lobbying effort from Kenya and its African allies.
The court, set up to prosecute the gravest international crimes, has so far handed down just two convictions, both
against little-known African warlords.
It opened its first investigation outside the continent on Wednesday, with a probe of the 2008 Russia-Georgia war.
