Abstract: A fastening device with a rapid clamping mechanism for the fastening of transverse supports to railing bars of a vehicle roof rack. On the transverse support a housing of the fastening device can be fixed. The housing has a fixed clamping jaw which can rest from above on the railing bar. The housing also includes two housing walls which are directed vertically and are spaced from each other, between which a lever mechanism is arranged on pivot pins and on mounting pins which are fixed to the housing. By pivotal motion of a clamping lever of the lever mechanism, a lower clamping jaw of the lever mechanism is moved to enclose the railing bar relative to the fixed clamping jaw. Thus, the railing bar is clamped between the lower clamping jaw and the fixed clamping jaw.