BAGHDAD - Elsewhere, in the southern city of Basra, an Iraqi police convoy was ambushed late Thursday, killing four policemen and wounding one, said police Capt. Mushtaq Khazim.

The new surge of violence before an Oct. 15 referendum on Iraq's constitution has killed at least 194 people, including 13 U.S. service members, in the past five days.

The insurgents have vowed to wreck the referendum, whose passage is crucial to prospects for starting a withdrawal of American troops.

Al-Qaida in Iraq, the country's most feared insurgent group, has declared "all-out war" on the Shiite majority that dominates Iraq's government. Moderate Sunni Arab leaders have urged their community to reject the constitution, saying it will fragment Iraq and leave them weak compared to Shiites and Kurds.

U.S. Ambassador Zalmay Khalilzad has been struggling to negotiate changes to the charter in hopes of winning Sunni Arab support, and senior U.S. officials in Washington have said they are confident that Iraq's draft constitution will be approved. But those officials also have said that if the constitution is defeated, Iraq could descend into anarchy.

Today, a car bomb exploded in a bustling vegetable market in the mostly Shiite city of Hillah, killing at least nine people and wounding 30, said police Dr. Ahmed Sami at the local hospital. The vehicle was parked when it detonated at about 9:30 a.m. in the city 60 miles south of Baghdad.

As Iraqi police and soldiers sealed off the Al-Sharia vegetable market, emergency workers lifted wounded victims and dead bodies into ambulances from streets covered with pools of blood and shattered vegetable stands.

In Iraq, the weekend is today and Saturday, and before heading to services at mosques midday today, the Muslim day of worship, many Iraqis shop in their local markets.

On Thursday, three suicide attackers exploded near-simultaneous car bombs in the heart of the bustling, mainly Shiite town of Balad, 50 miles north of the capital, killing at least 99 people and wounding 124, police and hospital officials said.

Apparently aimed at killing a large number of Shiite civilians, the string of bombings started just before sunset Thursday 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.

Most of the fatalities were civilians, though the wounded included the police chief and four officers, said Dr. Qassim Hatam, the director of Balad hospital.

New information about the Balad attack also emerged today, when police said the insurgents had hit a police checkpoint in the city with six mortar rounds at the same time, killing one civilian. U.S. soldiers based there returned the fire and detained an Iraqi suspect from a nearby home after finding traces of explosives on his body, the military said.

Insurgents in Iraq often have attacked forces and civilians racing to the scene of suicide car bomb explosions with mortar or machine gun fire.

Also Thursday, the U.S. military announced the deaths of five U.S. soldiers a day earlier 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 U.S. service members who have died since Iraq's war started in March 2003, according to an Associated Press count.

Until Thursday, however, Balad, the site of a major U.S. military air base, had seen few major attacks.

In Washington, the top American commander in Iraq said Thursday that the process of withdrawing U.S. 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," Gen. 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 instability.

Sunnis make up only 20 percent 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.

Sunni leaders complain the constitution does not emphasize Iraq's unity and Arab character. They say its federal system - which would allow Shiites in the south and Kurds in the north to form mini-states - will leave Sunnis in a weak middle region, cheated of oil resources.