BAGHDAD — Hundreds of young Iraqi men gripped by religious and nationalistic fervor streamed into volunteer centers Saturday across Baghdad, answering a call by the country’s top Shiite cleric to join the fight against Sunni militants advancing in the north.

Dozens climbed into the back of army trucks, chanting Shiite slogans and hoisting assault rifles, pledging to join the nation’s beleaguered security forces to battle the Sunni group known as the Islamic State of Iraq and the Levant, which has launched a lightning advance across the country.

“By God’s will, we will be victorious.” said one volunteer, Ali Saleh Aziz. “We will not be stopped by the ISIL or any other terrorists.”

The massive response to the call by the Iranian-born Grand Ayatollah Ali al-Sistani, issued via his representative Friday, comes as sectarian tensions are threatening to push the country back toward civil war in the worst crisis since U.S. forces withdrew at the end of 2011.

Fighters from the al-Qaida splinter group, drawing support from former Saddam Hussein-era figures and other disaffected Sunnis, have made dramatic gains in the Sunni heartland north of Baghdad after overrunning Iraq’s second-largest city of Mosul on Tuesday.

Soldiers and policemen have melted away in the face of the lightning advance, and thousands have fled to the self-rule Kurdish region in northern Iraq.

On Saturday, insurgents seized the small town of Adeim in Diyala province after Iraqi security forces pulled out. Adeim is about 60 miles north of Baghdad.

Jawad al-Bolani, a lawmaker and former Cabinet minister close to Prime Minister Nouri al-Maliki, said a military offensive was underway Saturday to drive the insurgents from Tikrit, Saddam’s hometown north of Baghdad.