We present a detailed 1.2 mm continuum and CS spectral line study of a large sample of 69 massive star forming regions in very early stages of evolution, most of them prior to building up an ultracompact H II region. The continuum data show a zoo of different morphologies and give detailed information on the spatial distributions, masses, column densities, and average densities of the whole sample. Fitting the radial intensity profiles shows that three parameters are needed to describe the spatial distribution of the sources: constant emission from the center out to a few arcseconds radius followed by a first power-law intensity distribution, which steepens farther outside into a second power-law distribution. The inner flat region is possibly caused by fragmentation of the large-scale cores into smaller subsources, whereas the steeper outer power-law distributions indicate finite sizes of the cores. Separating the sources into subsamples suggests that in the earliest stages prior to the onset of massive star formation, the intensity radial distributions are rather flat, resembling the structure of intensity peaks in more quiescent molecular clouds. Then in the subsequent collapse and accretion phase the intensity distributions become centrally peaked, with steep power-law indices. In this evolutionary stage the sources show also the broadest C³⁴S line width. During the following phase, when ultracompact H II regions evolve, the intensity power-law radial distributions flatten out again. This is probably caused by the ignited massive stars in the center which disrupt the surrounding cores. The mean inner power-law intensity index m_i (I ~ r) is 1.2, corresponding to density indices p (n ~ r^-p) of 1.6. In total, the density distributions of our massive star formation sites seem to be not too different from their low-mass counterparts, but we show that setting tight constrains on the density indices is very difficult and subject to many possible errors. The local densities we derive from CS calculations are higher (up to 1 order of magnitude) than the mean densities we find via the millimeter continuum. Such inhomogeneous density distribution reflects most likely the ubiquitous phenomenon of clumping and fragmentation in molecular clouds. Line width-mass relations show a departure from virial equilibrium in the stages of strongly collapsing cores.