  The temperature dependence of the magnetic field distribution in the mixed
superconducting phase of randomly oriented MgB$_2$ powder was probed by $%
^{11}$B NMR spectroscopy. Below the temperature of the second critical ($B_{%
{c2}}$) field, $T_{{c2}}\approx 27$K, our spectra reveal two NMR signal
components, one mapping the magnetic field distribution in the mixed
superconducting state and the other one arising from the normal state. The
complementary use of bulk magnetization and NMR measurements reveals that
MgB$_2$ is an anisotropic superconductor with a $B_{c2}^c<2.35$ Tesla
anisotropy parameter $\gamma\approx 6$.
