This paper is the first part of a series of three in which we present experimental and analytical data covering the development of entry profiles, normal and shear stress fields, velocity fields and the in-process extensional viscosity measurement of branched and linear polyolefins. Here, we report accurate in-process measurements of the natural flow profiles of selected branched and linear polyolefins obtained from screw-driven extrusion flows through an abrupt contraction geometry. Recirculation areas, vortex centres and detachment lengths were quantified and their dependence on flow rate and process temperature studied. Significant recirculation areas were observed for the branched melts. Both the size and the development of these features were dependent on flow rate and temperature. Recirculation areas of the linear polymers were less prominent and independent of flow conditions. Correlation has been observed between rheological characteristics of the low density polyethylenes and their entry behaviour.