  Some gamma-ray bursts (GRBs) have an afterglow in the tera-electronvolt (TeV)
band, but the early onset of this afterglow has not been observed. We report
observations with the Large High Altitude Air Shower Observatory of the bright
GRB 221009A, which serendipitously occurred within the instrument field of
view. More than 64,000 photons (above 0.2~TeV) were detected within the first
3000 seconds. The TeV photon flux began several minutes after the GRB trigger,
then rose to a flux peak about 10 seconds later. This was followed by a decay
phase, which became more rapid at $\sim 650\,{\rm s}$ after the peak. The
emission can be explained with a relativistic jet model with half-opening angle
$\sim 0.8^\circ$, consistent with the core of a structured jet. This
interpretation could explain the high isotropic energy of this GRB.
