The formation processes and the evolutionary stages of high-mass stars are poorly understood compared to low-mass stars. Large-scale surveys are needed to provide an unbiased census of high column density sites which can potentially host precursors to high-mass stars. Here we use the ATLASGAL survey covering 420 sq. degree of the Galactic plane at 870 micron; and use the MRE-GLC method to identify the population of embedded sources throughout the inner Galaxy. We identify in total 10952 compact sub-millimeter sources with fluxes above 5 sigma. Completeness tests show that our catalogue is 97% complete above 5 sigma and >99% complete above 7 sigma. We correlate this sample with mid-infrared point source catalogues (MSX at 21.3 micron and WISE at 22 micron) and determine a lower limit of ~33% that are associated with embedded protostellar objects. We note that the proportion of clumps associated with mid-infrared sources increases with increasing flux density, achieving a rather constant fraction of ~75% of all clumps with fluxes over 5 Jy/beam being associated with star-formation. Examining the source counts as a function of Galactic longitude we are able to identify the most prominent star forming regions in the Galaxy. From the fraction of the likely massive quiescent clumps (~25%) we estimate a formation time-scale of ~7.25+/-2.50 x 10^4~yr for the deeply embedded phase before the emergence of luminous YSOs. Such a short duration for the formation of high-mass stars in massive clumps clearly proves that the earliest phases have to be dynamic with supersonic motions.