I was a very young kid when I saw this movie.  I was born in 1975 so it had to have been made sometime after that.  I can only remember very few random details.  I hope I am not mixing memories of more than one movie.  Here is what I remember:

1. Setting – warm season, small town or desert-like town.

2. People in a bomb shelter (or storm cellar or basement)

3. Approximately 3-4 men in clown masks with weapons