The death toll rose from 40 to 56 from a - one of Iraq's deadliest attacks this year.
In eastern Baghdad, another roadside bombing wounded three pilgrims. A second bomb that went off a few minutes later about 70 yards away wounded a traffic policeman riding to the scene on his motorcycle.
The suicide bomber went after travelers enjoying tea and refreshments Sunday in a tent near Iskandariyah, 30 miles south of Baghdad. The blast killed at least 56 people and wounded 68, according to police and Babil health department director Dr. Mahmoud Abdul-Rida, driving the total number of pilgrims to 63 in two days.
Extremists had attacked another group of pilgrims with guns and grenades hours earlier in the predominantly Sunni Baghdad neighborhood of Dora, killing three and wounding 49, Iraqi military spokesman Maj. Gen. Qassim Atta said. He said the extremists fired from a mosque at the pilgrims and that a counterattack killed five of them, while two were captured.
U.S. and Iraqi forces have increased the number of checkpoints and imposed car bans and other measures in major Shiite cities to protect the worshippers traveling to Karbala, the burial site of Imam Hussein, the grandson of the Prophet Muhammad, who died in a seventh-century battle nearby and became one of Shiite Islam's most revered figures. Ceremonies will culminate in Karbala Wednesday to commemorate the end of the 40-day mourning period following the anniversary of his death
Major Shiite events have frequently been targeted in the past by suspected Sunni insurgents led by al Qaeda in Iraq in their drive to stoke sectarian violence.
Recent commemorations - including the Ashoura festival in mid-January to mark Imam Hussein's death - have passed without major bloodshed amid an overall decline in violence across Iraq.
But the pilgrims who walk for days to reach the shrine of Hussein are vulnerable despite the increased security.
Suicide attacks and car bombings are frequently blamed on al Qaeda in Iraq, but U.S. Col. Tom James, commander of the 4th Brigade, 3rd Infantry Division, which is responsible for the area around Sunday's attack, said it was too early to say who was behind that bombing.
Sunni leaders denounced the bombing, with hard-line politician Adnan al-Dulaimi's bloc blaming it on foreigners "aiming to create sectarian strife and to destabilize the country."
Meanwhile, Turkish troops fired dozens of salvos of artillery shells across the Iraqi border on Monday, a day after the military confirmed that a Turkish helicopter crashed in Iraq and eight military personnel were killed during a cross-border ground operation against Kurdish rebels.
The sound of the artillery fire from a distance could be heard in this border town of Cukurca. Several military bases that support the incursion into Iraq are on its outskirts, and artillery units have been positioned on hilltops overlooking Iraq.
Turkey began the ground operation Thursday to target autonomy-seeking Kurdish rebels who attack Turkey from hideouts on the Iraqi side of the border.
The guerrillas said Sunday they shot down a Turkish military helicopter near the Turkish-Iraqi border.
Turkey's military said technicians were inspecting the wreck to determine why the helicopter crashed near the border. It was not clear if any of the reported casualties were on board.
The military did not specify on its Web site whether the eight fatalities were troops or pro-government village guards, local residents who are familiar with the terrain and accompany the military on operations against the rebels. NTV said three village guards had been slain, but did not say when or where they died.
Turkey's military said Monday it had killed 41 more separatist Kurdish rebels in clashes in northern Iraq, raising the guerrilla death toll in its cross-border operation to 153.
The incursion is the first confirmed Turkish military ground operation in Iraq since the U.S.-led invasion that toppled Saddam Hussein in 2003.
The rebels of the Kurdistan Workers' Party, or PKK, are fighting for autonomy in predominantly Kurdish southeastern Turkey and have carried out attacks on Turkish targets from bases in the semiautonomous Kurdish region of northern Iraq. The conflict started in 1984 and has claimed as many as 40,000 lives.