At least seven people have been killed and 30 injured in a bomb explosion in Iraq's holy city of Karbala.
Sistani's spokesman said the bomb might be an assassination bid
The blast at the gate to a major Shia shrine, the Imam Hussein mausoleum, was the first serious attack in the city for several months.
An aide to Iraq's most senior Shia cleric, Grand Ayatollah Ali Sistani, was said to be among the wounded.
It came as campaigning for elections in January began and interim PM Iyad Allawi declared his candidacy.
Sheikh Abdul Mehdi Karbalai, the local representative of Ayatollah Sistani, was said to be among those hurt in Wednesday's attack.
A spokesman for Ayatollah Sistani told al-Jazeera television station the bomb was probably an attempt to assassinate the cleric.
Mr Karbalai was reported to be seriously injured and several of his bodyguards were among those killed in the explosion.
At least 85 people died and 230 were wounded in March when co-ordinated explosions near Karbala's main mosque targeted Shias who were celebrating a holy ritual.
Wednesday's blast came on the first day of campaigning for elections scheduled next month but it was unclear if there was any political motive.
A senior American general admitted that insurgency in Iraq "has become more effective".
Air Force Lt Gen Lance Smith said explosive attacks on US supply lines were slowing military operations and hindering the country's reconstruction.
On 30 January, voters will elect a 275-member assembly that will appoint a government and draft a constitution.
Allawi will be backed at the polls by a 240-member list of candidates
Speaking on the first official day of campaigning, Prime Minister Iyad Allawi announced he would stand at the polls backed by a 240-member list of candidates from his Iraqi National Accord party intended to have a broad appeal.
Electoral authorities say about 80 political blocs have registered to take part in the poll, including Sunni Muslim blocs.
They had threatened to boycott the vote amid fears it could be disrupted by the violent insurgency that continues to rage in Sunni areas.
BBC correspondent Peter Greste in Baghdad says it is hard to overstate the scale of the mountain that Iraq's democracy still has to climb.
The authorities must set up the physical electoral infrastructure across the country, including in places where the insurgency is still highly active.
Scheduled for 30 Jan
Voters to elect 275-member transitional assembly
Kurds also to pick 111-member autonomous parliament
Campaigning begins 15 Dec
230 parties running in about 80 blocs
Proportional representation based on party lists
Candidates must by over 30 and one third must be women
Militias, ex-top Baathists and current army officials barred
Balloting to take place in some 9,000 polling stations
More than 230 parties and groups, gathered into about 80 blocs or alliances, have to try to sell their messages amid the violence and chaos.
The campaigning begins a day after Mr Allawi announced that leaders in Saddam Hussein's regime would go on trial for crimes against humanity and war crimes as early as next week.
The first to appear would be Ali Hassan al-Majid - better known as "Chemical Ali".