AT least 20 Egyptian policemen were injured today when 1000 Christians staging a sit-in at the main Coptic cathedral in Cairo threw rocks at them. Roads leading to the cathedral, the seat of Pope Shenuda III, head of the Coptic Church, were sealed off, as the police tried to restore calm and restrain a protest over the disappearance of a priest's wife. The protesters claimed the woman was abducted by her Muslim boss and forced to convert to Islam.