The televised speech by Moqtada al-Sadr came only days after he announced he was withdrawing from politics, Middle East Online reported.
Sadr, known for his criticism of the U.S.-led invasion of Iraq in 2003, was no less relentless in slamming the government led by Prime Minister Nouri al-Maliki.
"Politics became a door for injustice and carelessness, and the abuse and humiliation of the rule of a dictator and tyrant who controls the funds, so he loots them, " Sadr said, "and the cities, so he attacks them, and the sects, so he divides them."
Iraqis must vote in the April elections "in a major way, so that the government does not fall into the hands of the dishonest," he added.
Noting that he was now dedicating himself "to Iraq and to Islam," the cleric reiterated his weekend statement he was removing himself from politics and his political movement, which holds numerous seats in Parliament and six cabinet posts.
The action was aimed at "preserving the honorable reputation of Sadr" and ending "all the corruptions that occurred or which are likely to occur."
Sadr had commanded the Mahdi Army, which battled U.S. troops, but has since suspended his fighters' activities. He is now reportedly concentrating on the study of Islam.