Laurent Akoun, the No. 2 man in the Ivorian Popular Front, was arrested following a rally about 60 miles east of Abidjan, said party official Sebastien Dano Djedje in a statement to Bloomberg News. Akoun was said to be being questioned by military authorities.
The Ivorian Popular Front is the opposition party loyal to Gbagbo. The International Criminal Court had said there are "reasonable grounds" to believe Gbagbo bears criminal responsibility for crimes against humanity carried out after a post-election crisis in 2010.
Ivory Coast was pushed to the brink of civil war following disputed 2010 presidential elections that saw Alassane Ouattara emerge victorious over Gbagbo. Ivorian leaders said they believed Gbagbo loyalists were behind some of the attacks in the country, though his allies deny the allegations.
Human rights group suspect forces loyal to Ouattara likely committed atrocities to the post-election conflict, however.