Twenty-three Congolese soldiers and seven civilians pleaded not guilty on Wednesday at the trial over an explosion at a munitions dump in Brazzaville which killed almost 300 people.

The group has been charged with arson and harming state security over the March 2012 disaster in the Congolese capital.

"Saying 'no' is not a strategy. When you are innocent, it is what you must say. They declared their innocence because they are," defence lawyer Patrick Delbar told AFP.

The explosion at the munitions dump of the Congolese army camp in capital's eastern district of Mpila had left 282 people dead and more than 2,300 injured. It also flattened hundreds of houses, leaving 17,000 homeless.

On the second day of the public hearings, defence lawyers also raised several objections to the proceedings, including querying the competence of the criminal court to hear the case.