The attack in Iskandariyah, south of Baghdad, was the second of the day against pilgrims traveling to the holy city of Karbala. The pilgrimage marks Arbaeen, the 40th day following the anniversary of the martyrdom of Imam Hussein, one of two revered Shiite figures buried there.

The suicide bomber detonated at a tent where pilgrims stop to eat and drink, police said.

At least 40 people were killed and 60 were wounded, the U.S. military said.

“The blast devastated the entire tent,” which was up by Prime Minister Nouri al-Maliki’s Shiite Dawa political party, local official Saleh al-Massoudi said.

Iraqi police and U.S. troops quickly responded to the attack, which occurred on a two-lane highway outside a residential area south of Iskandariyah, 30 miles south of Baghdad, the U.S. military said. It said about 42,000 pilgrims had previously traveled through the area without incident.

Earlier, extremists attacked another group of pilgrims with guns and grenades in the predominantly Sunni Baghdad neighborhood of Dora, killing three and wounding 36, police said.

The attacks heightened tension around Arbaeen, when millions of pilgrims descend on Karbala, about 50 miles south of the capital.

Earlier in the day, a steady stream of pilgrims - some carrying green, black or red banners - walked along a highway out of Baghdad en route to the shrine. Among them were many families, including black-robed women and children.