  Compton scattering provides invaluable information on the underlying Fermi
surface (FS) and is a powerful tool complementary to angle-resolved
photoemission spectroscopy and quantum oscillation measurements. Here we
perform high-resolution Compton scattering measurements for
La_{2-x}Sr_{x}CuO_{4} with x=0.08 (Tc=20K) at 300K and 150K, and image the
momentum distribution function in the two-dimensional Brillouin zone. We find
that the observed images cannot be reconciled with the conventional hole-like
FS believed so far. Instead, our data imply that the FS is strongly deformed by
the underlying nematicity in each CuO_2 plane, but the bulk FSs recover the
fourfold symmetry. We also find an unusually strong temperature dependence of
the momentum distribution function, which may originate from the pseudogap
formation in the presence of the reconstructed FSs due to the underlying
nematicity. Additional measurements for x=0.15 and 0.30 at 300K suggest similar
FS deformation with weaker nematicity, which nearly vanishes at x=0.30.
