  (Abridged) Aims. To investigate the first stages of the process of high-mass
star formation, we selected a sample of massive clumps previously observed with
the SEST at 1.2 mm and with the ATNF ATCA at 1.3 cm. We want to characterize
the physical conditions in such sources, and test whether their properties
depend on the evolutionary stage of the clump.
  Methods. With ATCA we observed the selected sources in the NH3(1,1) and (2,2)
transitions and in the 22 GHz H2O maser line. Ammonia lines are a good
temperature probe that allow us to accurately determine the mass and the
column-, volume-, and surface densities of the clumps. We also collected all
data available to construct the spectral energy distribution of the individual
clumps and to determine if star formation is already occurring, through
observations of its most common signposts, thus putting constraints on the
evolutionary stage of the source. We fitted the spectral energy distribution
between 1.2 mm and 70 microns with a modified black body to derive the dust
temperature and independently determine the mass.
  Results. The clumps are cold (T~10-30 K), massive (M~10^2-10^3 Mo), and dense
(n(H2)>~10^5 cm^-3) and they have high column densities (N(H2)~10^23 cm^-2).
All clumps appear to be potentially able to form high-mass stars. The most
massive clumps appear to be gravitationally unstable, if the only sources of
support against collapse are turbulence and thermal pressure, which possibly
indicates that the magnetic field is important in stabilizing them.
  Conclusions. After investigating how the average properties depend on the
evolutionary phase of the source, we find that the temperature and central
density progressively increase with time. Sources likely hosting a ZAMS star
show a steeper radial dependence of the volume density and tend to be more
compact than starless clumps.
