  We present $BVRI$ observations of the optical counterpart to the Gamma-Ray
Burst (GRB) 990510 obtained with the Las Campanas 1.0-m telescope between 15
and 48 hours after the burst. The temporal analysis of the data indicates
steepening decay, independent of wavelength, approaching asymptotically
$t^{-0.76\pm 0.01}$ at early times ($t\ll 1 day)$ and $t^{-2.40\pm 0.02}$ at
late times, with the break time at $t_0=1.57\pm 0.03 days$. GRB 990510 is the
most rapidly fading of the well-documented GRB afterglows. It is also the first
observed example of broad-band break for a GRB optical counterpart. The optical
spectral energy distribution, corrected for significant Galactic reddening, is
well fitted by a single power-law with $\nu^{-0.61\pm 0.12}$. However, when the
$B$-band point is dropped from the fit, the power-law becomes $\nu^{-0.46\pm
0.08}$, indicating a possible deviation from the power-law in the spectrum,
either intrinsic or due to additional extinction near the source or from an
intervening galaxy at $z=1.62$. Broad-band break behavior broadly similar to
that observed in GRB 990510 has been predicted in some jet models of GRB
afterglows, thus supporting the idea that the GRB energy is beamed, at least in
some cases.
