The Collaborative International Dictionary of English v.0.48:

Populace \Pop"u*lace\, n. [F. populace, fr. It. popolaccio,
   popolazzo, fr. popolo people, L. populus. See People.]
   The common people; the vulgar; the multitude, --
   comprehending all persons not distinguished by rank, office,
   education, or profession. --Pope.
   [1913 Webster]

         To . . . calm the peers and please the populace.
                                                  --Daniel.
   [1913 Webster]

         They . . . call us Britain's barbarous populaces.
                                                  --Tennyson.
   [1913 Webster]

   Syn: Mob; people; commonalty.
        [1913 Webster]

WordNet (r) 3.0 (2006):

populace
    n 1: people in general considered as a whole; "he is a hero in
         the eyes of the public" [syn: populace, public,
         world]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "populace":
   Everyman, John Doe, Public, body politic, bourgeoisie, canaille,
   citizenry, common man, commonage, commonalty, commoners,
   commonwealth, community, community at large, constituency, crowd,
   cultural community, dwellers, estate, everybody, everyman,
   everyone, everywoman, folk, folks, general public, gentry,
   habitancy, hoi polloi, inhabitants, linguistic community, masses,
   men, mob, multitude, nation, nationality, peasantry, people,
   people at large, people in general, persons, plebeians, plebes,
   polity, population, proletariat, public, rabble,
   ragtag and bobtail, rank and file, society, speech community,
   state, third estate, throng, whole people, working class, world,
   you and me