Abstract: We study two dimensional path integral Lefschetz thimbles, i.e. the possible path integration contours. Specifically, in the examples of the O(N) and (Formula preaented.) models, we find a large class of complex critical points of the sigma model actions which are relevant for the theory in finite volume at finite temperature, with various chemical potentials corresponding to the symmetries of the models. In this paper we discuss the case of the O(2m) and the (Formula preaented.) models in the sector of zero instanton charge, as well as some solutions of the O(2m + 1) model. The (Formula preaented.)-model for all instanton charges and a more general class of solutions of the O(N)-model with odd N will be discussed in the forthcoming paper.