  We present the results of prompt optical follow-up of the electromagnetic
counterpart of the gravitational-wave event GW170817 by the Transient Optical
Robotic Observatory of the South Collaboration (TOROS). We detected highly
significant dimming in the light curves of the counterpart (Delta g=0.17+-0.03
mag, Delta r=0.14+-0.02 mag, Delta i=0.10 +- 0.03 mag) over the course of only
80 minutes of observations obtained ~35 hr after the trigger with the T80-South
telescope. A second epoch of observations, obtained ~59 hr after the event with
the EABA 1.5m telescope, confirms the fast fading nature of the transient. The
observed colors of the counterpart suggest that this event was a "blue
kilonova" relatively free of lanthanides.
