  Recent interest in topological nature in condensed matter physics has
revealed the essential role of Berry curvature in anomalous Hall effect (AHE).
However, since large Hall response originating from Berry curvature has been
reported in quite limited materials, the detailed mechanism remains unclear at
present. Here, we report the discovery of a large AHE triggered by a
pressure-induced magnetic phase transition in elemental $\alpha$-Mn. The AHE is
absent in the non-collinear antiferromagnetic phase at ambient pressure,
whereas a large AHE is observed in the weak ferromagnetic phase under high
pressure despite the small averaged moment of $\sim 0.02 \mu_B$/Mn. Our results
indicate that the emergence of the AHE in $\alpha$-Mn is governed by the
symmetry of the underlying magnetic structure, providing a direct evidence of a
switch between a zero and non-zero contribution of the Berry curvature across
the phase boundary. $\alpha$-Mn can be an elemental and tunable platform to
reveal the role of Berry curvature in AHE.
