Abstract
Generalized symmetries with arbitrary functions of time t for the well known 2+1-dimensional integrable model, Kadomtsev-Petviashvili (KP) equation, are found by means of the extended mastersymmetry approach. Then an explicit and simple constructive formula for the symmetries of the KP equation is derived directly from the symmetry definition equation, without using complicated recursion operators. All the known symmetries appear as special cases of those obtained in this paper. The general infinite-dimensional Lie algebra constituted by these symmetries is also given.