"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," his representative announced on his behalf during today prayers in the city of Karbala.
"He who sacrifices for the cause of defending his country and his family and his honour will be a martyr," he added.
The elderly Sistani, who rarely appears in public, is highly influential in the Shiite Muslim world and is adored by millions.
Security forces have so far failed to halt the drive, with some fleeing after throwing away their uniforms and abandoning their positions.
Shiite Prime Minister Nuri al-Maliki has already announced that the government will arm and equip citizens who volunteer to fight.