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 \delta < -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 \sim 0.5 pc , a mass of \sim 320 ~ { } M _ { \odot } for a dust temperature T _ { d } = 30 K , an H _ { 2 } density of 9.5 \times 10 ^ { 5 } cm ^ { -3 } , and a surface density of 0.4 g cm ^ { -2 } . The median values are 0.4 pc , 102 ~ { } M _ { \odot } , 4 \times 10 ^ { 4 } cm ^ { -3 } , and 0.14 g cm ^ { -2 } , respectively . The mean value of the luminosity-to-mass ratio , L / M \simeq 99 ~ { } L _ { \odot } / M _ { \odot } , suggests that the sources are in a young , pre-ultracompact H ii 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 \sim 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 \sim 100 ~ { } M _ { \odot } is best fitted with a power-law dN / dM \propto M ^ { - \alpha } with \alpha = 2.1 , consistent with the Salpeter ( [ 1955 ] ) stellar IMF , with \alpha = 2.35 . On the other hand , the mass function of clumps with masses 10 ~ { } M _ { \odot } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } % \hbox { $ < $ } } } M \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } % \hbox { $ < $ } } } 120 ~ { } M _ { \odot } is better fitted with a power law of slope \alpha = 1.5 , more consistent with the mass function of molecular clouds derived from gas observations .