The incident happened in the bullring in Sangüesa, a town in Navarra, northern Spain as it celebrated its annual fiesta.
Someone in the crowd filmed the moment the bulls charged towards each other and collided full force with their heads. The impact caused instantaneous death and the animals fell to the ground.
WARNING: This video contains shocking footage.
Both bulls had been brought from a ranch in Avila and were to be killed by matadors in the ring. But their death occurred as the bulls entered the ring after Saturday's encierro, when daredevils run with the bulls from a pen on the outskirts of town to the bullring itself.
Earlier this month thousands of Spaniards took to the streets to demand an end to the centuries-old but controversial tradition of bullfighting.