[en] We have synthesized Mm (Mischmetal)-doped polycrystalline La_0.7Ca_0.3MnO₃ manganite by standard solid state reaction route and explored its structure and magnetotransport properties. Mm is a natural mixture of rare earth elements La, Ce, Pr and Nd. All Mm-doped samples crystallize in orthorhombic unit cell. At lower Mm doping the samples are single phase and at higher doping levels small amount of unreacted CeO₂ is observed. The lattice parameter and unit cell volume decrease with increasing Mm content due to decrease in average A-site radii. The decrease in A-site radii also causes reduction in ferromagnetism and metallicity. All samples possess significant MR in the vicinity of T_C, which increases on Mm doping. For Mm = 0.35 a huge magnetoresistance as high as ∼30% is observed at a low magnetic field of ∼3 kOe and this nearly doubles to ∼63% at ∼10 kOe. The improved MR in these materials is due to the existence of multivalent ions La, Mm and Ca, and the resulting presence of Mn²⁺, Mn³⁺ and Mn⁴⁺. This results in simultaneous operation of two competing interactions embodying FM double exchange between Mn²⁺-Mn³⁺ and Mn³⁺-Mn⁴⁺, and AFM superexchange between Mn²⁺-Mn²⁺ and Mn³⁺-Mn³⁺. In addition, the substitution of Mm also creates disorder at La-site as well as Mn-site. Thus, the simultaneous presence of two competing interactions and inhomogeneities induced disorder are prominent in enhancing the MR in the present case