Purely growing sawtooth precursors have been identified in high current, high-density discharges of the Frascati Tokamak Upgrade. On the basis of the observed helical structure and growth rate, these precursors are identified as the early non-linear stage of the m = 1 instability in the semicollisional regime. The precursor stage is followed by a fast collapse during which the growth rate of plasma displacement increases by an order of magnitude. During the precursor, a transition from semicollisional to collisionless reconnection regimes occurs, as the non-linear resistive width falls below the collisionless skin depth; such a cross-over is proposed as the trigger mechanism for the fast collapse phase. This simple scheme, in which the crossing of a linear stability boundary triggers the precursor, while a non-linear effect triggers the fast collapse, emerges in plasmas where the diamagnetic rotation frequency is smaller than the precursor growth rate. Diamagnetic effects complicate the picture as they reduce precursor growth rate, but the basic mechanisms are likely to be the same in all cases.