BAGHDAD, Iraq (AP) - At least 31 people were killed in a bomb attack Monday targeting poor Shiites seeking work as laborers, as the U.S. military announced the death of the 100th servicemember killed in Iraq this month.

The explosion in the sprawling Shiite slum of Sadr City tore through food stalls and kiosks at about 6:15 a.m. (0315 GMT), cutting down men who gather there each morning hoping to be hired as construction workers. At least 51 people were also injured, according to police Maj. Hashim al-Yasiri.

Sadr City, is a stronghold of the Mahdi Army loyal to radical Shiite cleric Muqtada al-Sadr, and has been the scene of repeated bomb attacks by suspected al-Qaida fighters seeking to incite Shiite revenge attacks and drag the country into full-blown civil war.

Ali Abdul-Ridha, injured on his head and shoulders, said he was waiting for a job with his brother and about 100 others when he heard a massive explosion and "lost sight of everything."

He said the area had been exposed to attack because U.S. and Iraqi forces had driven Mahdi fighters who usually provide protection into hiding.

"That forced Mahdi Army members, who were patrolling the streets, to vanish," the 41-year-old Abdul-Ridha said from his bed in al-Sadr Hospital, his brother lying beside him asleep.

However, Falih Jabar, a 37-year old father of two boys, said the Mahdi Army was responsible for provoking extremists to attack civilians in the neighborhood of 2.5 million people.

"We are poor people just looking to make a living. We have nothing to do with any conflict," said Jabar, who suffered back wounds. "If (the extremists) have problems with the Mahdi Army, they must fight them, not us," he added.

The U.S. and Iraqi military have kept a tight cordon around Sadr City since a raid there last week in search of an alleged Shiite death squad leader, who was not found.

The last major bombing in Sadr City occurred on Sept. 23 when a bomb hidden in a barrel blew up a kerosene tanker and killed at least 35 people waiting to stock up on fuel for the Muslim holy month of Ramadan.

Along with rising civilian casualties, October is already the fourth deadliest month for American troops since the war began in March 2003. The other highest monthly death tolls were 107 in January 2005; a reported 135 in April 2004, and 137 in November 2004.

The U.S. military identified the latest casualty as a Marine assigned to Regimental Combat Team 5. It said he died in combat Sunday in Anbar province west of Baghdad, a hotbed of the Sunni resistance to U.S. forces and their Iraqi government allies. The Marine's name was being withheld pending notification of next of kin.

Violence has rebounded with a vengeance following a brief lull during last week's Muslim holy days that followed the end of Ramadan, coinciding with U.S. efforts to bring Sunni insurgents into a reconciliation process and an embarrassing public squabble with Prime Minister Nouri al-Maliki over a schedule for achieving breakthroughs in security and political goals.

Political tensions deepened further on Sunday when Vice President Tariq al-Hashemi, the country's ranking Sunni politician, threatened to resign if al-Maliki did not move swiftly to eradicate militia groups.

Mohammed Shaker, a key aide to al-Hashemi, said the threat was intended to send a message to the government over the rising sectarian violence. "We cannot live with this situation indefinitely," Shaker said.

Al-Maliki depends heavily on the backing of a pair of Shiite political organizations and has resisted concerted American pressure to eradicate their private armies _ al-Sadr's Mahdi Army and the Badr Brigade, the military wing of Iraq's most powerful Shiite political bloc, the Supreme Council for the Islamic Revolution in Iraq, or SCIRI.

The gunmen, especially those of the Mahdi Army, are deeply involved in the sectarian killings that have brutalized Iraqis in Baghdad and central Iraq for months.

The militias have also infiltrated the predominantly Shiite security forces, who suffered around 300 deaths during Ramadan, mainly at the hands of Sunni insurgents but also in fighting between police and rival militia fighters.

At least 26 policemen were killed on Sunday, including 17 in one attack in the predominantly Shiite southern city of Basra. Gunmen dragged 15 policemen and two translators _ instructors at the Basra police academy _ off a bus at the edge of the city Sunday afternoon. Their bodies were found dumped throughout the city beginning about four hours later.

In the northern city of Kirkuk, a suicide bomber exploded a belt of dynamite he was wearing inside a police station at around noon on Monday, killing one policeman and one civilian and wounding 13 other people, police Brig. Sarhat Qadir said.

Three other policemen were killed when a car bomb hit a patrol Sunday night in northeastern Baghdad's Bunook neighborhood, police Lt. Ahmed Mohammed Ali said.

Unknown gunmen on Monday killed Essam al-Rawi, head of the University Professor's Union and a senior member of the hardline Sunni group, the Association of Muslim Scholars. One of his bodyguards was also killed.

The association, which is believed to have links to the insurgency, has boycotted elections and other aspects of the political process that seeks to bring stability and end rampant sectarian violence.

At least 154 university professors have been killed since the March 2003 U.S. invasion, Education Ministry spokesman Basil al-Khatib said Monday. Hundreds, possibly thousands, more are believed to have fled to neighboring countries, although al-Khatib said he had no specific numbers on those who have left the country.

While sectarian hatred is blamed for some of those attacks, professors have also been killed by students angered over poor grades or other grievances, or because of past membership in the now-outlawed Baath Party of former dictator Saddam Hussein.

The worsening violence in Iraq has become a pivotal issue in U.S. midterm elections next month, placing strains on relations between Washington and al-Maliki's shaky Shiite-dominated government.

The prime minister last week issued a series of angry statements, denouncing U.S. plans for a timeline to measure progress as infringing on Iraqi sovereignty and complaining to U.S. President George W. Bush over what he saw as imperious treatment from U.S. Ambassador Zalmay Khalilzad. The complaint followed an announcement by the Afghan-born Khalilzad that al-Maliki had agreed to set a timeline for progress on security and political reforms _ something the prime minister later denied.

In an interview with CNN on Sunday, Khalilzad said the squabble was a misunderstanding.

"That was a problem in how what I said was interpreted or translated to him and how it was played by some of the media here. What he understood as it was explained to him was that I had determined what issues and by when the Iraqis had to decide," the ambassador explained.

Saddam Hussein's chief lawyer walked out of court Monday after 12 of his requests were rejected, but the chief judge immediately appointed other attorneys to defend the deposed president.

The walkout came shortly after chief defense lawyer Khalil al-Dulaimi ended a monthlong boycott of the trial in which Saddam and six other defendants are charged with war crimes and crimes against humanity for a 1987-88 offensive against Iraq's Kurdish population.

Saddam already faces death by hanging in a separate case brought in connection with the killing of 148 Shiite villagers in Dujail after a 1982 assassination attempt against him. Iraqi court officials say a verdict in the first trial would definitely be handed down on Sunday, two days before the American election.