CAIRO – Egypt's military assumed responsibility Monday for protecting state institutions and maintaining security ahead of a Dec. 15 constitutional referendum, as the country braced for another round of mass demonstrations by the supporters of the country's Islamist president and the liberal opposition over the disputed charter.

The referendum on a contentious new constitution lies at the heart of a bitter political battle that has deeply polarized Egypt and triggered some of the worst street violence between backers and opponents of President Mohammed Morsi since he took power in June as the country's first democratically elected leader.

So far, Morsi has stood firm on the referendum, refusing to yield to opposition demands that he scrap the vote scheduled for Saturday. The opposition, meanwhile, was still trying to decide late Monday whether to boycott the referendum or rally Egyptians to vote "no" to the draft constitution, and hoping that a massive turnout for a rally Tuesday would force the president to cancel the balloting.