Respected Shia cleric Mohammed Bahr al-Uloum said Shi’ites, who make up the majority in Iraq, are committed to peaceful participation in next month’s election.
He urged Shi’ites to focus on the upcoming elections and not on avenging the bombings.
A spokesman for the movement led by the militant Shi’ite cleric Moqtada Sadr said civil war would be “hell”.
The deadly blasts came as insurgents stepped up attacks on the religious group ahead of next month’s historic elections.
Sunday’s bombings were the deadliest attacks on the Shi’ite community since last spring, when more than 180 people were killed in attacks on religious shrines in Karbala and Baghdad.
The bombs, detonated about two hours apart, went off near crowded bus stations in a seemingly coordinated attempt to cause as much bloodshed as possible among Shi’ites, a long-oppressed majority expected to dominate the vote on January 30.
The explosion in Najaf was near the golden-domed Imam Ali Mausoleum, one of the most revered spots in all of Shiite Islam.
Fourty-eight people died and 90 others were wounded, according to a local hospital official.
The blast gutted the busy commercial street of Hassan al-Bannat, packed with shops and medical clinics.
Police, who blamed the radical Islamist group Al-Qaeda for the attack, imposed a nighttime curfew, set up checkpoints around the city and raided homes of suspected insurgents.
Two hours later, 14 people were reported killed and 57 wounded when another car bomb exploded at a bus station at Karbala.
In the second deadly attack in the city in less than a week, many of the dead and injured were reported to be women and children.
The attack happened just a short distance from the tomb of Imam Hussein, the Muslim prophet Mohamed’s grandson and one of the most revered figures in the Shiite faith.
Witnesses said the car had tried to enter a police recruiting centre but the street was sealed off so it was driven instead into the nearby bus station, where it exploded in a ball of fire.
Last week a bomb apparently targeting Shi’ite cleric Abdul Mehdi al-Kerbalai exploded in Karbala as he was returning to his office after evening prayers.
Ten people, including four of Kerbalai’s bodyguards, were killed and more than 30 wounded in that attack.
Sunday’s attacks appeared designed to provoke sectarian conflict with Saddam Hussein’s long-dominant Sunni minority.
Officials have reported similar motives behind previous attacks in the cities.
The bombings raised fears of an upsurge in violence in the predominantly Shiite communities in southern Iraq, until now, enjoying a respite from the violence in central Iraq, home to the country’s disgruntled Sunni minority.
Iraq’s Shiites make up 60 percent of Iraq’s 25 million population and are expected to dominate the next government after the January 30 elections.
Shi’ite religious leaders say they won’t be provoked by the bombings and reject accusations by some secular opponents that they want to install an Iranian-style Shi’ite theocracy.
Many militant Sunnis, waging an insurgency against American occupation, fear domination by Shi’ites.
Insurgents have also made direct efforts to disrupt plans for the January poll.
In Baghdad, gunmen dragged three employees of Iraq’s Electoral Commission from a car and shot them dead in the street.
The car was set on fire and the bodies left lying near the burning wreckage.
US and Iraqi officials have warned that attacks will be stepped up in the run up to the election, Iraq’s first fully democratic poll in nearly half a century.