We have carried out the density--functional molecular--dynamics simulations to investigate the finite--temperature behavior as well as the geometric and electronic properties of pure Li clusters (Li$_{10}, Li$_{12}$) and Al--doped Li clusters (Li$_{10}$Al, Li$_{10}$Al$_2$). We find that addition of two Al impurities in Li$_{10}$ results in a substantial structural change, while the addition of one Al impurity causes a slight rearrangement of atoms. Introduction of Al--impurity in Li$_{10}$ establish a polar bond between Li and nearby Al atom(s), leading to a multicentered bonding, which weakens the Li--Li metallic bonds in the system. These weakened Li--Li bonds lead to a premelting feature to occur at lower temperature in clusters with impurity. In Li$_{10}$Al$_2$, Al atoms also form a weak covalent bond, which results in their dimer--like behavior in the system. This causes Al atoms not to `melt' till 800 K, in contrast to Li atoms showing a complete diffusive behavior above 400 K. Thus, although one Al impurity in Li$_{10}$ cluster does not change its melting characteristics significantly, two impurities results in `surface melting' only, whose motions are confined around dimer--like Al atoms.