We describe a systematic program aimed at identifying and characterizing candidate high-mass protostellar objects (HMPOs). Our candidate sample consists of 69 objects selected by criteria based on those established by Ramesh & Sridharan using far-infrared, radio continuum, and molecular line data. IRAS and Midcourse Space Experiment data were used to study the larger scale environments of the candidate sources and determine their total luminosities and dust temperatures. To derive the physical and chemical properties of our target regions, we observed continuum and spectral line radiation at millimeter and radio wavelengths. We imaged the free-free and dust continuum emission at wavelengths of 3.6 cm and 1.2 mm, respectively, searched for H₂O and CH₃OH maser emission, and observed the CO J = 2 → 1 line and several NH₃ lines toward all sources in our sample. Other molecular tracers were observed in a subsample. While dust continuum emission was detected in all sources, most of them show only weak or no emission at 3.6 cm. Where detected, the centimeter emission is frequently found to be offset from the millimeter emission, indicating that the free-free and dust emissions arise from different subsources possibly belonging to the same (proto)cluster. A comparison of the luminosities derived from the centimeter emission with bolometric luminosities calculated from the IRAS far-infrared fluxes shows that the centimeter emission very likely traces the most massive source, whereas the whole cluster contributes to the far-infrared luminosity. Estimates of the accretion luminosity indicate that a significant fraction of the bolometric luminosity is still due to accretion processes. The earliest stages of HMPO evolution we seek to identify are represented by dust cores without radio emission. Line wings due to outflow activity are nearly omnipresent in the CO observations, and the molecular line data indicate the presence of hot cores for several sources, where the abundances of various molecular species are elevated because of evaporation of icy grain mantles. Kinetic gas temperatures of 40 sources are derived from NH₃ (1, 1) and (2, 2) data, and we compare the results with the dust temperatures obtained from the IRAS data. Comparing the amount of dust, and hence the gas, associated with the HMPOs and with ultracompact H II (UCH II) regions, we find that the two types of sources are clearly separated in mass-luminosity diagrams: for the same dust masses, the UCH II regions have higher bolometric luminosities than HMPOs. We suggest that this is an evolutionary trend, with the HMPOs being younger and reprocessing less (stellar) radiation in the IR than the more evolved UCH II regions. These results indicate that a substantial fraction of our sample harbors HMPOs in a pre-UCH II region phase, the earliest known stage in the high-mass star formation process.