The attack in the eastern city that was the cradle of last year's uprising which overthrow Moamer Kadhafi took place as protesters demonstrated outside the NTC office, witnesses and an NTC member told AFP.
"People were protesting in front of the NTC office and then one grenade was thrown at the office," he said on condition of anonymity.
An AFP correspondent at the scene said he did not see any apparent damage to the NTC office or any casualties.
Witnesses said the device was thrown when wounded former rebels who helped topple Kadhafi were protesting at being "marginalised" in the new Libya.
Home-made bombs were regularly used by the former rebels during the conflict against Kadhafi last year, especially to attack checkpoints of the former leader's forces.
Protesters have been holding regular demonstrations in Benghazi for several weeks, accusing the NTC of lacking transparency and recruiting members to the body who were once seen as loyalists of the former regime.