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.

Mr. 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.

In a statement Sunday, liberal youth activists of the April 6 Movement rejected Mr. Morsi's new decree as an attempt at “manipulation” and “deception.” They called for more protests to stop the referendum and the draft constitution, which they fear will erode civil rights because it boosts the role of Islamic law in society.

Prominent Egyptian opposition figure Mohammed ElBaradei sent out a tweet Sunday, saying the fight against the constitution is about the “essence of the state, universal rights and values, and looking forwards, not backwards.”

An opposition coalition was due to meet later Sunday to decide on a formal response to President Morsi's new decree, which removed provisions of the previous decree that shielded all of his decisions from judicial review. Opposition groups had complained that the original measure gave the president dictatorial powers.

But, Mr. Morsi's latest decree said the courts remain barred from challenging the president's “constitutional declarations,” including Saturday's measure.

Egypt's military made its first public comment on the political crisis Saturday, urging both sides to resolve political differences through dialogue. It warned that continued confrontation risks plunging Egypt into a “dark tunnel leading to catastrophe” and vowed “not to allow” such an outcome.

Egyptian troops set up concrete barriers outside the presidential palace Sunday to prevent rival groups of liberals and Islamists from holding further rallies at the site. Street battles between the two sides have killed seven people in the capital in recent days.

In a rare move, Egyptian fighter jets also made low passes over central Cairo on Sunday. There was no immediate word from the Egyptian military on the reason for the overflights.