CAIRO — A truck bomb blasted the main security headquarters in Cairo on Friday, one of a string of four bombings hitting police in the Egyptian capital within a 10-hour period, killing six people. The most significant attack yet in the city fueled a furious backlash against the Muslim Brotherhood amid rising fears of a militant insurgency.

In the hours after the blast, angry residents – some chanting for the “execution” of Brotherhood members – joined police in clashes with the group’s supporters holding their daily street protests against the government. Smoke rose over the city and fighting around the country left 14 more people dead.

The mayhem on the eve of the third anniversary of 2011’s once hopeful revolution pointed to the accelerating, dangerous slide Egypt has taken since last summer’s military ouster of Islamist President Mohammed Morsi: a mounting confrontation between the military-backed government and Islamist opponents amid the escalating militant violence.

Saturday, the anniversary of the start of the 18-day uprising that ousted autocrat Hosni Mubarak, raised the potential for new violence, as both military supporters and the Islamists vowed to take to the streets with rival rallies.

After Friday’s blasts, interim Preisident Adli Mansour vowed to “uproot terrorism,” just as the government crushed a militant insurgency in the 1990s.

That could spell an escalation in the crackdown that the government has waged against Morsi’s Muslim Brotherhood since his July 3 ouster.