Ayatollah Sistani issued the call to arms to defend Iraq against the offensive spearheaded by the armed group the Islamic State of Iraq and the Levant (ISIL), which had gained more ground in the country overnight by seizing the strategically important towns of Jalulah and Saaiydiyah.
"Citizens who are able to bear arms and fight terrorists, defending their country and their people and their holy places, should volunteer and join the security forces to achieve this holy purpose," Ayatollah Sistani's representative announced on his behalf during Friday prayers in the Shiite shrine city of Karbala.
"He who sacrifices for the cause of defending his country and his family and his honor will be a martyr," he added.
The elderly cleric, who rarely appears in public, is highly influential in the Shiite Muslim world and is revered by millions.
Shortly after Sistani made his appeal, more than a thousand volunteers from Karbala made their way down to an army camp in the city of Taji, about 30km north of Baghdad.
The AP reported that they were set to take part in a one-day training course before being deployed to fight alongside Iraqi troops.
ISIL fighters have taken over a huge swathe of predominantly Arab territory in northern and north-central Iraq since launching their offensive in the country's second city, Mosul, on Tuesday. They have since pushed south into ethnically divided Diyala province, towards Baghdad.
On Friday, they were fighting pro-government forces near Muqdadiyah, just 80km from Baghdad city limits.