The disclosure relates to a free-electron laser system and a method for generating a packet of relativistic electrons capable of propagating in a first propagation direction (Oz), and a device for generating an undulator beam capable of interacting with the packet of relativistic electrons. In the system, the undulator beam results from combining, at an interaction area through which the propagation direction (Oz) of the packet passes, at least two laser beams propagating in different directions and each of which has at least one non-zero component in a plane orthogonal to the propagation direction (Oz) of the packet. The disclosure also relates to a method for generating a free-electron laser beam involving trapping and guiding a packet of relativistic electrons injected into an interaction area and implementing such a free-electron laser system.