  During our optical monitoring of UV Ceti, iconic late-type flaring star, with
high temporal resolution using the Russian 6-m telescope in 2008 we detected a
giant flare with the amplitude of about 3 magnitudes in $U$-band. Near flare
maximum more than a dozen of spike bursts have been discovered with triangular
shapes and durations from 0.6 to 1.2 s and maximal luminosities in the range
$(1.5-8)\times10^{27}$ erg~s$^{-1}$. For the half of these events the linear
polarization exceeds 35% with significance better than $5\sigma$. We argue that
these events are synchrotron emission of electron streams with the energies of
several hundred MeV moving in the magnetic field of about 1.4 kG. Emission from
such ultrarelativistic (with energies far exceeding 10 MeV) particles is being
routinely observed in solar flares, but has never been detected from UV Ceti
type stars.
  This is the first ever detection of linearly polarized optical light from the
UV Ceti-type stars which indicates that at least some fraction of the flaring
events on these stars are powered by a non-thermal synchrotron emission
mechanism.
