•The fluctuation mechanism of the internal friction in Ba0.8Sr0.2TiO3 was discovered.•The internal friction peak increases under a dc electric field.•Losses are due to the interaction of charged point defects with domain walls.•Doping with La atoms changes the internal friction. The temperature dependences of internal friction and Young’s modulus in ferroelectric Ba0.8Sr0.2TiO3 ceramics under bending oscillations were investigated at frequencies of the several hertz. The effect of the heating rate and the frequency of oscillations on the internal friction peak height near the Curie temperature was studied. The fluctuation model of low-frequency internal friction is used to explain the obtained dependences and to estimate the nuclei volume of a new phase arising at a phase transition. After a dc electric field switched on in the ferroelectric phase, the time decrease of internal friction was observed, which had an exponential form. The addition of 0.2 mass. % La in Ba0.8Sr0.2TiO3 led both to an increase in the internal friction level in the ferroelectric phase and to a decrease in the internal friction maximum near the Curie temperature. In the first case, this was explained by a decrease in the interaction energy of charged point defects with ferroelectric domain walls, and in the second case, by the pinning of domain walls by lanthanum ions.