Aug 17 (Reuters) - Canada’s main stock index opened lower on Tuesday, as heavyweight energy shares were weighed down by extended losses in oil prices on global demand concerns. * At 9:32 a.m. ET (13:32 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 107.63 points, or 0.53%, at 20,375.79. Reporting by Amal S in Bengaluru; Editing by Sriraj Kalluvila Our Standards: The Thomson Reuters Trust Principles.