The first overtly sectarian battle of the new war unfolding for control of Iraq came as thousands of heavily armed Shiites paraded defiantly through the streets of Baghdad, responding to a call to arms from their most revered cleric.

Men young and old piled into taxis, trucks, police vehicles and minibuses and drove noisily through the streets, honking horns and waving an assortment of weapons, including rocket-propelled grenades, high-powered sniper rifles, Kalashnikov assault rifles and handguns.

“Daiish we will never surrender,” chanted a group of young men piled on the back of a pickup truck, using an acronym for the al-Qaeda-inspired Islamic State of Iraq and Syria (ISIS), which carried out a stunningly swift seizure of large swaths of northern Iraq last week.