A three-dimensional unsteady flow separation in the straight diffuser of a model bulb turbine is investigated with planar two-component PIV measurements near the wall. The turbine is operated in two selected conditions that give rise to separation zones of different size and shape. The blockage effect induced by separation leads to a sudden drop in turbine efficiency and power extraction. The separation front fluctuates significantly both in location and in shape with no periodicity. From conditionally averaged results, it is deduced that the mean separation front is tilted azimuthally and that the mean separation skin friction line is composed of a saddle point on the diffuser side with one of its branches running along the diffuser bottom. Vortices and separation front critical points are analysed with POD-reconstructed instantaneous velocity fields. Separation surface vortices are generally bigger and stronger than turbulent vortices within or outside the separation zone, which suggests that different roll-up mechanisms are involved. The separation surface is irregular and is populated near the wall by a succession of foci and saddle points.