Lots of civilians have willingly taken up arms to fight, and at that point, I have to say they cease being civilians. Whether this is because they wanted to go and overthrow the regime, or because they thought they were going to be attacked one way or another, I don't know, but I don't pretend to know, either.
This is a very imporant point -- there were also reports on NPR that it was precisely because
of this that the UN may have trouble distinguishing who's who on the ground. Our mission is not to bomb the hell out of the pro-Gaddafi forces, the mission is to protect civilians, regardless of who they support. The rebels are such a mixed bag that many of them appear to be civilians both in and out of battle.
In regards to mercenaries, it's pure speculation at this point with little evidence (as far as I'm aware) pointing in either direction to say for sure. I wouldn't put it past him, as it's the same tactic used by Mubarak, but then, Mubarak's hired guns fell apart very quickly and did not have near the strength of Gaddafi's army, in comparison.
...On a semi-related note, anyone else wondering where Gaddafi's virgin babe deathsquad is doing during this whole thing?