  We report results from a Giant Metrewave Radio Telescope (GMRT) monitoring
campaign on the black hole X-ray binary V404 Cygni during its 2015 June
outburst. The GMRT observations were carried out at observing frequencies of
1280, 610, 325 and 235 MHz, and extended from June 26.89 UT (a day after the
strongest radio/X-ray outburst) to July 12.93 UT. We find the low-frequency
radio emission of V404 Cygni to be extremely bright and fast-decaying in the
outburst phase, with an inverted spectrum below 1.5 GHz and an intermediate
X-ray state. The radio emission settles to a weak, quiescent state $\approx 11$
days after the outburst, with a flat radio spectrum and a soft X-ray state.
Combining the GMRT measurements with flux density estimates from the
literature, we identify a spectral turnover in the radio spectrum at $\approx
1.5$ GHz on $\approx$ June 26.9 UT, indicating the presence of a synchrotron
self-absorbed emitting region. We use the measured flux density at the turnover
frequency with the assumption of equipartition of energy between the particles
and the magnetic field to infer the jet radius ($\approx 4.0 \times 10^{13}$
cm), magnetic field ($\approx 0.5$ G), minimum total energy ($\approx 7 \times
10^{39}$ ergs) and transient jet power ($\approx 8 \times 10^{34}$ erg
s$^{-1}$). The relatively low value of the jet power, despite V404 Cygni's high
black hole spin parameter, suggests that the radio jet power does not correlate
with the spin parameter.
