  High-resolution Hubble Space Telescope (\textit{HST}) optical observations
have been used to perform the deepest photometric study of the poorly studied
Galactic globular cluster NGC 6284. The deep colour-magnitude diagram (CMD)
that we obtained reaches 6 magnitudes below the main sequence turn-off. We
provide the first determination of the gravitational centre ($C_{\rm grav}$)
and density profile of the system from resolved stars. $C_{\rm grav}$ is
significantly offset (by $1.5-3''$) from the values in the literature. The
density profile shows the presence of a steep central cusp, unambiguously
indicating that the cluster experienced the core-collapse phase. Updated values
of the structural parameters and relaxation times of the system are provided.
We also constructed the first high-resolution reddening map in the cluster
direction, which allowed us to correct the evolutionary sequences in the CMD
for the effects of differential reddening. Isochrone fitting to the corrected
CMD provided us with new estimates of the cluster age, average colour excess,
metallicity, and distance. We find an absolute age of $13.3 \pm 0.4$ Gyr, an
average colour excess $E(B-V) = 0.32 \pm 0.01$, a metallicity [Fe/H]$= -1.36
\pm 0.01$, and a true distance modulus $(m-M)_0 = 15.61 \pm 0.04$ that sets the
cluster distance at $13.2 \pm 0.2$ kpc from the Sun. The superb quality of the
CMD allowed a clear-cut identification of the Red Giant Branch (RGB) bump,
which is clearly distinguishable along the narrow RGB. The absolute magnitude
of this feature turns out to be $\sim 0.2$ mag fainter than previous
identification.
