We have theoretically studied light-induced many-body dynamics of metallic nanoparticles (NPs) under the focused laser beam and the thermal fluctuations in liquid medium at room temperature. The effect of surface ionization is also taken into account. The random motion of NPs by collisions of medium molecules can be suppressed with increasing the laser intensity, and the transition from a diffusive state to metastable states of metallic NPs is possible. Obtained results will provide a novel way to control the phase transitions in assembled structure of NPs under the balance of optically-modulated interparticle forces and thermal fluctuations. Such a mechanism is significantly different from a conventional gas-liquid-solid transition based on spontaneous interparticle forces because it is based on the transient anisotropic force controlled by light.