  Shock breakout emission is light that arises when a shockwave, generated by
core-collapse explosion of a massive star, passes through its outer envelope.
Hitherto, the earliest detection of such a signal was at several hours after
the explosion, though a few others had been reported. The temporal evolution of
early light curves should reveal insights into the shock propagation, including
explosion asymmetry and environment in the vicinity, but this has been hampered
by the lack of multiwavelength observations. Here we report the instant
multiband observations of a type II supernova (SN 2023ixf) in the galaxy M101
(at a distance of 6.85+/-0.15 Mpc), beginning at about 1.4 hours after the
explosion. The exploding star was a red supergiant with a radius of about 440
solar radii. The light curves evolved rapidly, on timescales of 1-2 hours, and
appeared unusually fainter and redder than predicted by models within the first
few hours, which we attribute to an optically thick dust shell before it was
disrupted by the shockwave. We infer that the breakout and perhaps the
distribution of the surrounding dust were not spherically symmetric.
