Prosecutors said Said oversaw a prison in the capital Bangui where inmates were beaten and tortured.
"I plead not guilty to all charges and all situations," Said, dressed in a dark grey suit, told the judges.
The mostly Muslim Seleka militia groups seized power in 2013-2014 as part of the Central African Republic's long-running civil war, ousting then-President Francois Bozize.
In the Central African Republic's capital Bangui a viewing of the live-stream of the opening of the trial was organized in a courtroom.
"For us victims, we can say that (the opening) is a great relief, and we are paying close attention to the trial at the ICC, " Francine Evodie Ndemade, president of an association for victims of military conflict in CAR, told Reuters.
ICC prosecutor Karim Khan told judges at the start of the trial that the Seleka ruled "by fear, by terror". In the period of crimes charged, Khan said Said "wielded immense power" in the prison he ran.
According to Khan, Said "actively hunted down civilians" and brought them to the prison knowing "what nightmare awaited them under his control" including torture, being held in "putrid conditions" in an underground cell known as the hole without food or water.
The ICC has charged leaders of both the Seleka and opposing Christian militias known as "anti-balaka". Said is the first alleged Seleka member to go on trial.
In Bangui a woman in her thirties who was captured and imprisoned by Seleka fighters in 2014 and asked to remain anonymous, said she welcomed the Said trial.
She added that she hoped the ICC would also look into reparations "because we, the victims, have suffered too much."
The Central African Republic has been mired in violence since a coalition of mostly northern and predominantly Muslim rebels known as Seleka, or "Alliance" in the Sango language, seized power in March 2013. Their rule gave rise to the opposing anti-balaka Christian militias.
Monday's hearing was adjourned and the trial will continue on Tuesday with the defence opening statement.
(Reporting by Stephanie van den Berg and Judicael Yongo in Bangui; Editing by Frank Jack Daniel, Angus MacSwan, William Maclean)
By Stephanie van den Berg