As four ministers from Mohammed Morsi’s government resigned in the wake of the mass protests demanding his resignation, the Muslim Brotherhood announced Monday afternoon that it would “defend itself” from anti-government protesters, in the wake of the trashing of its headquarters in Cairo earlier Monday.

Egypt seemed to once again be spinning out of control, after millions of Egyptians hit the streets Sunday night demanding that Morsi, who has been in office for just a year, resign the presidency. Sixteen people have been killed in violence between pro- and anti-government protesters, eight of them outside the Brotherhood’s headquarters, while over 700 have been injured. The protests, observers said, dwarfed anything that was aimed at former Egyptian leader Hosni Mubarak, and police and the army said the potential for mass violence was extremely high.

Another huge protest is set for Cairo on Monday night, but after the veiled threats by the Muslim Brotherhood Monday that it would set up “self-defense units” against anti-government protesters in the wake of the storming of its headquarters, observers said they expected a bloody night, with far fiercer fighting between the two sides than has been seen until now.

As if to underscore just how tenuous the situation was, four of Morsi’s ministers – those of the tourism, parliamentary affairs, communications and environmental ministries – quit their posts Monday afternoon. The resignations are expected to increase the pressure on Morsi to step down himself, or at least call new elections, a move he has absolutely refused to consider so far.

A report earlier Monday said that a revolutionary group active in the anti-government protests had issued an ultimatum to Morsi to resign or call new elections by Tuesday at 5 PM local time. If he did not do so, the group said, it would organize a mass march to the Presidential Palace, “in order to fulfill the will of the people.”