Intermediate-mass black holes (BHs) bridging the gap between stellar mass (MBH ≤ 100 M⊙) and supermassive BHs (M_BH ≥ 106 M⊙) are still elusive [1]. Recent James Webb Space Telescope discoveries of supermassive BHs in the early Universe (z ∼ 7-10) call for new pathways for their formation [2, 3] when the Universe was less than a billion years old, requiring intermediate-mass BHs as their seeds [4]. Here, we report the observations of a remarkable optical transient event CRTS-CSS161010 J045834-081803 (CSS161010 hereafter) 0.4” from the centre of a dwarf galaxy with a stellar mass log(M∗/M⊙)= 8.135+0.087 −0.079 M⊙. The bright and extremely fast-evolving light curves of CSS161010 share many similarities with those of the rare, luminous, fast blue optical transients. Spectroscopically, its evolution is unprecedented. The spectra show blueshifted, broad hydrogen emission lines indicating velocities of up to 10% of the speed of light. We show that CSS161010’s observed properties can naturally be explained by the tidal disruption of a star by an intermediate-mass BH with M BH = 10 3.6±0.1 M⊙. Further observations of events similar to CSS161010 can pinpoint quiescent intermediate-mass BHs in low-redshift galaxies and constrain their masses, shedding light on the formation of massive BHs in the early Universe.