Rashi on Deuteronomy 27:12/רש"י על דברים כ״ז:י״ב

1לברך את העם [THESE SHALL STAND UP …] TO BLESS THE PEOPLE — The procedure was as is found in Treatise Sotah 32a: Six tribes ascended the top of Mount Gerizim and the other six the top of Mount Ebal, the priests, the Levites and the Ark remaining below in the midst (i.e. in the valley between). The Levites turned their faces towards Mount Gerizim and began to recite the blessing: “Blessed be the man that does not make any graven or molten image etc.”, and both these and those (the tribes on Mount Gerizim and those on Mount Ebal) answered “Amen”! Then they turned their faces towards Mount Ebal and began to recite the curse, saying: “Cursed be the man who maketh any graven [or molten image]” — and so in the case of all of them (the curses set forth here) till the last: “Cursed be he that does not uphold [all the words of this law to do them]” (cf. Rashi on Deuteronomy 11:29).