These human towers are called "muixeranga", and they are built in important celebrations. This weekend we are celebrating Our Lady of he Forsaken, who is the patron saint of the city. Other years, Muxerangues have been built but, as far as I know, this is the first one to be erected inside the basilica.
@bigmxx Thanks for commenting, it is dangerous, but they train a lot. It's amazing to see it live.
Ian