Egyptian opposition activists have called for more mass protests against President Mohamed Morsi's plan to hold a referendum on a new constitution largely shaped by his Islamist allies.Morsi issued a decree late Saturday insisting the December 15 referendum will go ahead, while also making a concession to the opposition by canceling parts of a November 22 decree that granted himself sweeping powers. He made the announcement after talks with other political figures earlier in the day, but most opposition groups boycotted the dialogue.