It's believed the aide, Sheik Abdul Mahdi al-Karbalayee, was the intended target, reports CBS News Correspondent Cami McCormick. Iraq's Shiite population is hoping for a strong showing in the upcoming elections, and Sunni insurgents have been accused in the past of attacking Shiite holy sites.
The bomb at Karbala's Imam Hussein Shrine came on the first day of campaigning for Iraq's Jan. 30 election, for which al-Sistani helped put together a powerful coalition of leading Shiite parties that is expected to do well in the vote.
The election, Iraq's first national elections since Saddam Hussein's ouster, will be held to seat an assembly that will then appoint a government and draft the country's constitution.
Iraq's interim leader, Prime Minister Ayad Allawi, announced his candidacy.
In other developments:
Allawi's long-expected decision to join the polls was the highlight of what was a low-key beginning to the campaigning period, which many fear will be targeted by insurgents opposed to the elections.
"The terrorists do not want these elections to take place, they have been very clear about so I think we should anticipate that the violence will in fact escalate between now and the 30th of January," former Iraq administrator L. Paul Bremer told CNN.
Karbala, 50 miles south of Baghdad, was the scene of heavy fighting in April between the militia of anti-U.S. cleric Muqtada al-Sadr and multinational forces. At the time, the U.S. military accused fighters loyal to the rebel cleric of firing on American forces from the mosque.
The Imam Hussein Shrine was also one of a number of Shiite holy sites in Karbala and Baghdad targeted in March, when coordinated bombs and suicide blasts hit pilgrims attending a religious festival, killing at least 181.
Shaalann, who has previously accused Tehran of interfering in Iraq's affairs, said Iranian and Syrian intelligence agents, plus former operatives from Saddam Hussein's security forces, are cooperating with Jordanian militant al-Zarqawi "to run criminal operations in Iraq."
Al-Zarqawi's al Qaeda in Iraq group is believed to be leading a brutal campaign of hostage-takings, beheadings and bombings that victimize both Americans and Iraqis. U.S. officials have offered a $25 million bounty for al-Zarqawi.
Iran and Syria have rejected U.S. and Iraqi claims they support Iraq's insurgency, but Damascus has said it is unable to fully close its long, porous border with its neighbor.
Shaalan's comments may have been aimed at trying to stir up sentiment against the United Iraqi Alliance, a leading coalition of mainly Shiite candidates, some with close ties to Iran, which is expected to do well in the vote.
He took a swipe at an architect of the 228-member coalition and leading member, nuclear physicist Hussain al-Shahristani, describing him as the "leader of an Iranian list" that wants to Iraq to be run similar to its Shiite-dominated neighbor.
Allawi announced he is backed in the election by a 240-member list of candidates meant to highlight his appeal to Iraq's diverse and sometimes fractious ethnic and religious groups.
Surrounded by women and men clad in tribal garb, clerical turbans and smart suits, Allawi pledged to work for national unity and move away from "religious and ethnic fanaticism." He did not say how many members were on his list.
"By depending on God, and with a firm determination and based on strong confidence in the abilities of our people, we are capable of confronting the difficulties and challenges and of making a bright future for our honorable people," Allawi said.
Allawi said his party would push for the eventual withdrawal of multinational forces.
"Rebuilding the army and the forces of national safety enable us to work on asking for the final withdrawal of the multinational forces from our beloved country according to a set timetable," he said.
On Tuesday, Allawi announced Iraq will bring top figures of Saddam's ousted regime to court next week for the first time since they appeared before a judge five months ago, and formal indictments could be issued next month.
Analyst Sajjan Gohel of the London-based think tank Asia Pacific Foundation said putting former leaders of a middle east country on trial for war crimes enters "uncharted territory."
"We're dealing with an area which is still very unclear, unheard of in the Middle East, where former rulers of a country are put on trial for human rights abuses, so it's going to be a very fascinating time," he told CBS News.
He said investigators are hoping that trying "Chemical Ali" first may strengthen the case against Saddam Hussein.