We investigate convection in a cylindrical Rayleigh-Bénard cell with radius-to-height ratio Γ = 1/2. The cell is subjected to impulsive spin-up about its vertical axis. We use TLC (thermochromic liquid crystal) for temperature field measurements and PIV (particle image velocimetry) for the velocity reconstruction of the transition in the range of Rayleigh numbers R from 5 × 107 to 5 × 108 and dimensionless rotation rates Ω from 0 up to 8 × 104. The initial (at rest) and the final (in steady rotation) states of the system are those of turbulent convection. The most persistent transient feature is a sharply defined ringlike pattern characterized by a decrease in temperature, axial velocity directed downward and high azimuthal shear. The latter leads to formation of an azimuthally regular structure of Kelvin-Helmholz vortices. During the final stage of the transition, this vortical structure loses azimuthal regularity and an irregular pattern of vortices characteristic of turbulent rotating convection forms.