  The progenitors of high-mass stars and clusters are still challenging to
recognise. Only unbiased surveys, sensitive to compact regions of high dust
column density, can unambiguously reveal such a small population of
particularly massive and cold clumps. Here we study a flux limited sample of
compact sources from the ATLASGAL survey to identify a sample of candidate
progenitors of massive clusters in the inner Galaxy. Sensitive mid-infrared
data at 21-24 $\mu$m from the WISE and MIPSGAL surveys were explored to search
for embedded objects, and complementary spectroscopic data were used to
investigate their stability and star formation activity. Based on such
ancillary data we identify an unbiased sample of infrared-quiet massive clumps
in the Galaxy that potentially represent the earliest stages of massive cluster
formation. An important fraction of this sample consists of sources that have
not been studied in detail before. Comparing their properties to clumps hosting
more evolved embedded objects, we find that they exhibit similar physical
properties in terms of mass and size, suggesting that infrared-quiet massive
clumps are not only capable of forming high-mass stars, but likely also follow
a single evolutionary track leading to the formation of massive clusters. The
majority of the sources are not in virial-equilibrium, suggesting collapse on
the clump scale. This is in line with the low number of infrared-quiet massive
clumps and earlier findings that star formation, in particular for high-mass
objects is a fast, dynamic process. We propose a scenario in which massive
clumps start to fragment and collapse before their final mass is accumulated
indicating that strong self-gravity and global collapse is needed to build up
rich clusters and the most massive stars.
