In an ongoing effort to identify and study high-mass protostellar candidates we have observed in various tracers a sample of 235 sources selected from the IRAS Point Source Catalog, mostly with δ < -30 degr, with the SEST antenna at millimeter wavelengths. The sample contains 142 Low sources and 93 High, which are believed to be in different evolutionary stages. Both sub-samples have been studied in detail by comparing their physical properties and morphologies. Massive dust clumps have been detected in all but 8 regions, with usually more than one clump per region. The dust emission shows a variety of complex morphologies, sometimes with multiple clumps forming filaments or clusters. The mean clump has a linear size of 0.5 pc, a mass of 320 M_⊙ for a dust temperature Td = 30 K, an H2 density of 9.5×105 cm-3, and a surface density of 0.4 g cm-2. The median values are 0.4 pc, 102 M_⊙, 4×104 cm-3, and 0.14 g cm-2, respectively. The mean value of the luminosity-to-mass ratio, L/M ≃ 99 L_⊙/M_⊙, suggests that the sources are in a young, pre-ultracompact Hii phase. We have compared the millimeter continuum maps with images of the mid-IR MSX emission, and have discovered 95 massive millimeter clumps non-MSX emitters, either diffuse or point-like, that are potential prestellar or precluster cores. The physical properties of these clumps are similar to those of the others, apart from the mass that is 3 times lower than for clumps with MSX counterpart. Such a difference could be due to the potential prestellar clumps having a lower dust temperature. The mass spectrum of the clumps with masses above M ∼ 100 M_⊙ is best fitted with a power-law dN/dM propto M-α with α = 2.1, consistent with the Salpeter (1955) stellar IMF, with α=2.35. On the other hand, the mass function of clumps with masses 10 M_⊙⪉ M ⪉ 120 M_⊙ is better fitted with a power law of slope α = 1.5, more consistent with the mass function of molecular clouds derived from gas observations.