The onset of rain caused several riders to veer off the track in a long right-hander, with the bikes colliding with each other as the rising riders tried to avoid the slipping ones.
One of the machines caught fire but the fire was quickly extinguished by the emergency services of the Portimao circuit (south of Portugal).
The accident occurred during the press conference of the first three riders of the MotoGP race which had ended an hour earlier.
The accident was broadcast live on TV screens in the press room and Fabio Quartararo, Johann Zarco and Aleix Espargaro expressed their amazement at the scene unfolding before their eyes.