A homicide bomber struck a busy commercial center in a major Shiite city south of Baghdad on Tuesday, killing at least 24 people and wounding dozens as the streets were packed with shoppers and people on their way to work, police and hospital officials said.

The explosion occurred at 9 a.m. in Hillah, according to provincial police, who said the driver of the tow truck detonated his payload in the middle of the Bab al-Mashhad district. Iraqi troops cordoned off the area while fire engines and ambulances rushed to the scene.

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Eassam Rashid, 32, was selling vegetables at his stall when the blast sent shrapnel over his head.

"I heard a tremendous explosion followed by a fire ball," he said. "Then nearby cars were set ablaze one by one and I saw four or five people struggling to get out of their burning cars."

Most of the 24 killed and 69 wounded in the blast suffered serious burns, said Ayad Abdul-Zahra of the Hillah general hospital.

Hillah, about 60 miles south of Baghdad, has been the site of some of the deadliest bombings, including a double homicide attack on March 6 that killed 120 people.

The attack came a day after at least 16 people died when four car bombs rocked the center of the capital. Three of the blasts took place in one 30-minute span.

Police, morgue and hospital officials reported a total of at least 59 people killed or found dead nationwide Monday, and the American military announced the deaths of three soldiers and a Marine. At least 3,636 members of the U.S. military have died since the Iraq war began in March 2003, according to an Associated Press count.

The continued fighting and deaths of Iraqis and American forces in the sixth month of the American bid to calm Baghdad and the center of the country illuminate the stubborn resistance to a political solution in Iraq.

The government and legislature are under heavy U.S. pressure to overcome sectarian differences and agree to measures aimed at promoting national unity as Americans are engaged in a fierce debate over calls to bring U.S. troops home from the unpopular war.

The U.S. and Iranian ambassadors to Iraq also began talks Tuesday in Baghdad in a bid to find ways to use their influence to bring stability to Iraq despite rising tensions over Washington's allegations that Tehran is fueling the violence and disputes over detainees.

Prime Minister Nouri al-Maliki opened the meeting with a statement welcoming the attendants at his headquarters in the heavily fortified Green Zone.

The meeting was closed to the media, but photos released by the Iraqi leader's office showed the participants sitting at three long tables for each delegation linked in triangular fashion and covered with white cloths. Al-Maliki was joined by Foreign Minister Hoshyar Zebari, while the U.S. delegation was headed by Ambassador Ryan Crocker and the Iranians by Hasan Kazemi Qomi.

Hundreds of demonstrators, meanwhile, marched in the predominantly Shiite district of Shaab in northern Baghdad to protest a U.S.-Iraqi barricade of Husseiniyah, a town on the capital's northeastern outskirts that is known as a Shiite militia stronghold. Police issued calls for residents to leave the town, and some said they were running out of food and fuel.

Protesters changed anti-American slogans and burned what appeared to be a hand-drawn American flag as they demanded an end to the blockade, access to the area for government rescue teams and compensation for families of any casualties.

The Shiite-dominated parliament has said al-Maliki should intervene to end the crackdown by U.S. and Iraqi forces on Husseiniyah. The town is dominated by the Mahdi Army, the militia loyal to radical cleric Muqtada al-Sadr, and straddles the highway to Baqouba, where U.S. forces are in the second month of a drive to cleanse that region of Al Qaeda in Iraq.

State-run Iraqiya television said the Husseiniyah blockade "would have serious consequences on people's lives there."

A 51-year-old woman resident, who would give her name only as Um Bassem, said police, apparently expecting a major outbreak of fighting, had issued calls for residents to leave Husseiniyah if they could.

"My husband offered to take us out and return to protect our house and belongings, but we refused to leave because we would be so worried about him," Um Bassem told an AP reporter in the area. She said food stocks were becoming low.

"We decided to stay home in two rooms at the back of the house. We can't leave because we have valuable things and we fear looters," she said.

Lt. Col. Michael Donnelly, spokesman for U.S. forces north of Baghdad, said American and Iraqi forces were now allowing "commercial vendors to bring food to the south of Husseiniyah. Civilians are authorized to walk to these vendors to buy food. Donkey carts may be used, but no vehicle movement is authorized. We are also allowing civilians that need medical aid, to walk to the Hamid Shaub Hospital for free treatment."

Trouble broke out in Husseiniyah when U.S. forces took small arms fire shortly before midnight Friday and ordered an airstrike on the building from which the gunmen were shooting. The military said helicopters fired missiles at the building and three gunmen fled to a second building.

U.S. aircraft then bombed the second structure, setting off at least seven secondary blasts believed caused by explosives and munitions stored inside the building, the military said, adding that Iraqi police told American forces six militants were killed and five wounded.

The military account contradicted reports from Iraqi police and hospital officials contacted by The Associated Press. Those officials said 18 civilians had been killed and 21 wounded in the attacks at 2 a.m. Saturday.

AP Television News videotape showed wounded women and children lying in hospital beds, and white pickup trucks carrying at least 11 bodies wrapped in blankets to the morgue. Men unloaded the bodies, including several that were small and apparently children.

Relatives said the dead were killed in the airstrike. The conflicting accounts could not be reconciled.

Donnelly said militant gunmen "are using civilians as protection and have no regard for the innocent.

"Currently there are berms (earthen barriers) placed to impede movement to/from the city by the militia group, who have fired on CF (Coalition Forces) over the past day(s). The intent of these berms remains unclear, but it is impeding movement in and out of the town for sure," he said in response to an e-mail asking for details.

Elsewhere, a joint U.S.-Iraqi force also conducted an operation in northeastern Baqouba that led to the detention of 16 suspected militants, including five teenagers, according to the Iraqi military.

Complete coverage is available in FOXNews.com's Iraq Center.