The back-reflected image of a 100 TW laser incident on a long scale length
plasma is measured. The plasma is deliberately preformed on a solid planar
target in a controlled way. Multiple highly intense spots are observed insi
de the original focal spot. These spots could be the experimental evidence
for the laser beam relativistic filamentation in the plasma; Three-dimensio
nal particle-in-cell (PIC) simulations for parameters close to the experime
ntal values are performed. The experimental observations and the filamentat
ion dynamics obtained in the PIC simulations are in a good agreement.