Polycrystalline silicon has been deformed in-situ in a high-voltage 1000 kV electron microscope (HVEM) at a temperature of 780 degrees C and at decreasing temperatures down to 530 degrees C. The generation of new dislocations and their motion have been observed. The dislocation velocity in a selected glide plane has been determined for five different temperatures. At 530 degrees C the velocities have been determined at different loads. From the measured data the stress exponent m=1.5+or-0.2 has been estimated. The activation energy of dislocation gliding was estimated from the temperature dependence of the applied tensile force to E=1.5+or-0.3 eV. The local shear stress has been determined from the curvature of segments of mobile dislocations which have been pinned temporarily. The results are discussed in comparison to parameters found for monocrystalline material.