Plasma waves contribute to many fundamental phenomena, including astrophysics$^{1}$, thermonuclear fusion$^{2}$ and particle acceleration$^{3}$. Such waves can develop in numerous ways, from classic Langmuir oscillations carried by electron thermal motion$^{4}$, to the waves excited by an external force and travelling with a driver$^{5}$. In plasma-based particle accelerators$^{3,6}$, a strong laser or relativistic particle beam launches plasma waves with field amplitude that follows the driver strength up to the wavebreaking limit$^{5,7}$, which is the maximum wave amplitude that a plasma can sustain. In this limit, plasma electrons gain sufficient energy from the wave to outrun it and to get trapped inside the wave bucket$^{8}$. Theory and numerical simulations predict multi-dimensional wavebreaking, which is crucial in the electron self-injection process that determines the accelerator performances$^{9,10}$. Here we present a real-time experimental visualization of the laser-driven nonlinear relativistic plasma waves by probing them with a femtosecond high-energy electron bunch from another laser-plasma accelerator coupled to the same laser system. This single-shot electron deflectometry allows us to characterize nonlinear plasma wakefield with femtosecond temporal and micrometre spatial resolutions revealing features of the plasma waves at the breaking point.