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The discovery of the spontaneous mode-locking of lasers^,, that is, the self-starting synchronous oscillation of electromagnetic modes in a cavity, has been a milestone of photonics allowing the realization of oscillators delivering ultrashort pulses. This process is so far known to occur only in standard ordered lasers and only in the presence of a specific device (the saturable absorber). We engineer a mode-selective pumping of a random laser formed by a self-assembled cluster of nanometric particles. We show that the random laser can be continuously driven from a configuration exhibiting weakly interacting electromagnetic resonances^, to a regime of collectively oscillating strongly interacting modes^,. This phenomenon, which opens the way to the development of a new generation of miniaturized and all-optically controlled light sources, may be explained as the first evidence of spontaneous mode-locking in …