Powerful car bomb blasts have killed and injured scores of civilians in the Iraqi Shia cities of Najaf and Karbala.
Officials have warned of more violence in the run-up to the poll
In Najaf, at least 48 people died and 90 were injured when a bomb exploded near the Imam Ali shrine, doctors said.
A similar explosion at a crowded bus station in Karbala left 13 dead and 30 injured, police said.
In Baghdad earlier in the day, three Iraqi election workers were shot dead. The violence comes six weeks before Iraq is due to hold elections.
The BBC's Caroline Hawley reports from Baghdad that the aim of the bombers - believed to be Sunni insurgents - is to kill as many Shias as possible and try to stir up sectarian trouble ahead of the 30 January poll.
Leading Shias have urged their supporters not to respond with retaliatory attacks.
"The Shias are committed not to respond with violence, which will only lead to violence. We are determined on elections," said one of the most respected Shia clerics, Mohammed Bahr al-Uloum.
And a spokesman for the Sadr movement led by the militant cleric, Moqtada Sadr, said civil war would be "hell".
US and Iraqi officials have said they expect the violence to increase in the run-up to the poll.
From his prison cell, former President Saddam Hussein has appealed to Iraqis not to take part in the elections.
He "urged the unity of his Iraqi people, regardless of their religious and ethnic creed, to confront US plans to divide their country on sectarian grounds," his lawyer Ziad Khasawneh said on Sunday.
In other developments:
- Two mortar rounds strike the Um al-Tubou Sunni Muslim mosque in Baghdad, injuring four security guards and shattering windows
- Iraqi insurgents issue a videotape showing 10 abducted Iraqis, whom they threaten to kill unless their American company leaves Iraq
- Iraqi police detain 45 men who illegally entered the country from neighbouring Iran
- US Defence Secretary Donald Rumsfeld promises to personally sign all future letters of condolences to families of service personnel killed in Iraq, after reports that a machine had been used to attach his signature
The attacks in Iraq's holy cities claimed the highest number of civilian deaths in a single day since July.
People had gathered in Najaf's Maidan Square - close to the revered Imam Ali mosque - for the funeral procession of a tribal sheikh when the bomb exploded.
Najaf police chief Ghalib Jazaari, who was standing with provincial governor Adnan Zorfi at the time, said he believed they were the targets, according to the Associated Press news agency.
A 15-year-old boy lost his left leg in the blast.
"I was working in the cafe when I heard a big explosion," Khalid Jabar told the AP.
"I felt that I was hit by an electric shock. Then I saw my left leg on the ground."
The attack in Karbala was a "suicide operation" outside an entrance to the main bus terminal, police there said.
Police spokesman Rahman Mashawi said: "This is a terrorist attack carried out by terrorist groups in order to destabilise the security in the city."
Witnesses said the car carrying the bomb tried to enter a police recruitment centre but when its driver found the street blocked off, the vehicle
ploughed into the bus terminal.