After all, what is the point in trying to get your message across if no one is there there to listen? And I am not just talking about getting other peace movement people there either. The hope is to inform and educate, to raise people's awareness, rather than just preaching to the converted.
The best way to attract people to your event is to publicize it through traditional and non-traditional means. Poster your local campus, coffee houses, and even businesses that attract people who may be interested (always ask permission before hanging a poster on private property). Community and college radio stations will sometimes allow organizations to announce events, without costing anything. Announcements in community newspapers also tend to be low cost or free.
The main thing to remember is to start early and gain momentum. Someone may ignore a poster the first time, but after the third or fourth time will start to take notice. Give people enough time to learn what your event is about, not just when and where.
Don't forget to send press releases to all the major and community media who may give your event coverage both prior and after it takes place.