In situ measurements of the dislocation mobility in polycrystalline silicon are carried out in a 1000 kV high voltage electron microscope (HVEM) using a heating-straining stage at temperatures between 530 and 780 degrees C. Most of the dislocations are found to move in the (111) (101) glide system. The effective shear stress (between 10 and 80 MPa) is determined from video recordings by analysing the radius of curvature of dislocation segments fixed at pinning points. Dislocation velocities of different dislocation types are averaged. The stress exponent m=1.6+or-0.2 and the activation energy E=(1.6+or-0.3) eV of dislocation glide are determined from the measured data. The results are discussed in comparison to parameters of monocrystalline material.