  Abridged: ATLASGAL is an unbiased 870 micron submillimetre survey of the
inner Galactic plane. It provides a large and systematic inventory of all
massive, dense clumps in the Galaxy (>1000 Msun) and includes representative
samples of all embedded stages of high-mass star formation. Here we present the
first detailed census of the properties (velocities, distances, luminosities
and masses) and spatial distribution of a complete sample of ~8000 dense clumps
located in the Galactic disk. We derive highly reliable velocities and
distances to ~97% of the sample and use mid- and far-infrared survey data to
develop an evolutionary classification scheme that we apply to the whole
sample. Comparing the evolutionary subsamples reveals trends for increasing
dust temperatures, luminosities and line-widths as a function of evolution
indicating that the feedback from the embedded proto-clusters is having a
significant impact on the structure and dynamics of their natal clumps. We find
88\,per\,cent are already associated with star formation at some level. We also
find the clump mass to be independent of evolution suggesting that the clumps
form with the majority of their mass in-situ. We estimate the statistical
lifetime of the quiescent stage to be ~5 x 10^4 yr for clump masses ~1000 Msun
decreasing to ~1 x 10^4 yr for clump masses >10000 Msun. We find a strong
correlation between the fraction of clumps associated with massive stars and
peak column density. The fraction is initially small at low column densities
but reaching 100\,per\,cent for column densities above 10^{23} cm^{-2}; there
are no clumps with column density clumps above this value that are not already
associated with massive star formation. All of the evidence is consistent with
a dynamic view of star formation wherein the clumps form rapidly and are
initially very unstable so that star formation quickly ensues.
