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Let $G$ be a non-compact simple Lie group with Lie algebra $\mathfrak{g}$.Denote with $m(\mathfrak{g})$ the dimension of the smallest non-trivial$\mathfrak{g}$-module with an invariant non-degenerate symmetric bilinear form.For an irreducible finite volume pseudo-Riemannian analytic manifold $M$ it isobserved that $\dim(M) \geq \dim(G) + m(\mathfrak{g})$ when $M$ admits anisometric $G$-action with a dense orbit. The Main Theorem considers the case $G= \widetilde{\mathrm{SO}}_0(p,q)$ providing an explicit description of $M$ whenthe bound is achieved. In such case, $M$ is (up to a finite covering) thequotient by a lattice of either $\widetilde{\mathrm{SO}}_0(p+1,q)$ or$\widetilde{\mathrm{SO}}_0(p,q+1)$.

On low-dimensional manifolds with isometric $\mathrm{SO}_0(p,q)$-actions