BAGHDAD, Iraq – Three suicide attackers exploded near-simultaneous car bombs in the heart of a bustling, mainly Shiite town yesterday, killing at least 60 people and wounding 70 amid a new surge of violence before an October 15 referendum on Iraq’s constitution.

Apparently aimed at killing a large number of Shiite civilians, the string of bombings started just before sunset when the first blast ripped through an open-air market crowded with Iraqis buying vegetables. The next bomb exploded at a bank just yards away, followed by a third on a nearby street of clothing shops.

Sunni insurgents have vowed to wreck the referendum, the passage of which is crucial to prospects for starting a withdrawal of American troops. Al Qaeda in Iraq has declared “all-out war” on the Shiite majority that dominates Iraq’s government, and moderate Sunni Arab leaders called on their community to reject the constitution, saying it will fragment Iraq and leave them weak compared to Shiites and Kurds.

The American ambassador has been struggling to negotiate changes to the charter in hopes of winning Sunni Arab support. Frustrating his efforts, Sunnis said American troops raided the homes of two Sunni leaders on Thursday, fueling their sense of alienation in the political process.

Also yesterday, the American military announced the deaths of five American soldiers Wednesday in a roadside bombing during combat in Ramadi, west of Baghdad, a hotbed of Iraq’s insurgency.

It was the deadliest single attack against American troops in more than a month, bringing to 1,934 the number of American service members who have died since Iraq’s war started in March 2003, according to an Associated Press count. More than 140 people, including 13 American service members, have been killed in the past four days.

Until yesterday, however, Balad – 50 miles north of Baghdad and the site of a major American military air base – had seen few major attacks.

The blasts left streets strewn with body parts and wounded as emergency vehicles rushed in. The attackers detonated their explosives-packed cars within minutes of each other, starting at 6:45 p.m. at the Masraf Street market, then at a nearby commercial avenue, Bint al-Hassan Street, police Lieutenant Ghafil Hassan said.

Most of the 60 casualties were civilians, though among the 70 wounded were the police chief and four officers, said Dr. Khaled al-Azawi of Balad Hospital. Lieutenant Hassan said the toll could reach 75 dead and 100 wounded, saying there were still victims at the scene.

In Washington, the top American commander in Iraq said yesterday that the process of withdrawing American troops depends greatly on the results of the referendum and elections set to follow if the constitution passes.

“The next 75 days are going to be critical,” General George Casey told the U.S. Senate Armed Services Committee. But Sunni Arab success in rejecting the constitution would set back the political process for months, prolonging Iraq’s political stability.

Sunnis make up only 20% of the population, but they could defeat the charter because of a loophole in voting rules: If two-thirds of voters in any three of Iraq’s 18 provinces vote “no,” the referendum fails – even if an overall majority approves. There are four provinces where Sunnis could potentially cross that margin.

America’s ambassador to Iraq, Zalmay Khalilzad, has been shuttling between all sides, trying to secure last minute changes to the draft, which parliament approved September 18 after tough negotiations.

He has met rejections from Shiites and Kurds on some proposed changes, and some Sunni officials said the proposals were still not enough.

According to Sunni officials, American forces raided the Baghdad homes of senior officials in a prominent Sunni political group, the Conference for Iraq’s People, Adnan al-Dulaimi and Harith al-Obeidi.

Mr. al-Dulaimi, 73, said soldiers in tanks and Humvees broke into his home at 2:30 a.m., put him and his family in a guest room and searched the house, arresting four of his bodyguards and confiscating weapons he said were licensed.

“It was if they were attacking a castle, not the home of a normal person who advises Iraq’s interim government and has called for reconciliation and renounced sectarianism,” said Mr. al-Dulaimi.

Mr. al-Dulaimi said the Americans were acting on a false tip that his bodyguards had links to insurgents. The American military said it conducted several raids in neighborhoods where the leaders live but could not identify the homes involved.