  We present a detailed magneto-optical investigation of the magnetic flux
penetration in polycrystalline MgB2 slabs made by direct reaction of B and Mg.
Our results unambiguously indicate a uniform, Bean critical state magnetization
behavior with almost no electromagnetic granularity. From the measured magnetic
flux profiles we were able to extract the temperature dependence of the
critical current density Jc(T). The Jc(T) value reaches 1.8x10^5 A/cm2 at 10K
and 0.12T, in good agreement with global magnetization measurements.
