BAGHDAD — A suicide bomber struck Shiite pilgrims as they were resting today during a days-long walk to a Shiite shrine, killing at least 40 people and wounding 60.

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 American military said.

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

Iraqi police and American 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 American 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.

In Karbala itself, crowds already choked the streets though the culmination of event is Wednesday. Four million pilgrims were already in the city as of a couple of days ago, police said.

Police Chief Raid Shakir Jawdat has said 40,000 police officers and military troops are being deployed during this period because Shiite holidays have frequently been targeted by suspected Sunni insurgents.

For example, a parked car loaded with explosives was discovered and put out of action near Karbala, one of several potential attacks that have already been averted, Chief Jawdat said.

Four million pilgrims were already in the city as of a couple of days ago, he said.

Elsewhere, extremists targeted American patrols in two separate attacks in northern Baghdad, one of which killed a soldier and wounded three other troops and a civilian, the military said, without naming the victims. The second bombing wounded three soldiers, the military said.

Mr. al-Maliki, meanwhile, flew to London for a second round of medical tests, about two months after having his heart checked during a previous visit, aides said.

A government spokesman, Ali al-Dabbagh, said Mr. al-Maliki, on his last visit over New Year’s, underwent a cardiac catheterization, a routine procedure in which a thin tube is inserted into an artery or a vein to check for heart problems.

“This trip is only for a checkup,” Mr. al-Dabbagh said. “There is nothing wrong with him. He was asked by the doctor to come back within six weeks, and that is why he is going.”

Mr. al-Maliki’s adviser, Yassin Majeed, also said today that the prime minister was in good health and the tests this time were merely to confirm the previous results.