July 11 (Reuters) - Iran's Supreme Leader Ayatollah Mojtaba Khamenei said on Saturday that avenging his slain predecessor and father was "the demand of the nation" and "must certainly" take place, according to a written message released on his Telegram account. Khamenei issued the message on the occasion of funeral ceremonies for his father, Ayatollah Ali Khamenei, held months after he was killed in the U.S.-Israeli airstrikes on February 28.