Story Highlights• NEW: Interior Ministry: Baghdad bombs kill 198, wound about 240
• NEW: 140 dead in Sadriya market, Baghdad's worst bombing
• NEW: U.S. soldier dies in Baghdad
• "Weakness of security measures" in Baghdad cited as reason for arrest
• British transfer security to Iraqis in southern Shiite province
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BAGHDAD, Iraq (CNN) -- An Iraqi army brigade commander was arrested Wednesday night after a string of bombings that killed nearly 200 people around Baghdad, most of them in a single attack at a central marketplace, Prime Minister Nuri al-Maliki's office announced.
The explosion at the Sadriya market, one of the Iraqi capital's oldest and busiest venues, killed 140 people and wounded about 150, an Iraqi Interior Ministry official told CNN. A statement from al-Maliki's office said the brigade commander was removed because of "the weakness of security measures put in place to protect civilians in Sadriya."
The marketplace attack was the worst of at least six bombings across Baghdad -- a particularly bloody day in a city wracked by sectarian warfare and a U.S.-Iraqi crackdown against insurgents. It was the worst bombing in the Iraqi capital since the 4-year-old war began, topping the February toll of 130 dead in a bombing in the same marketplace. (Watch as death toll climbs after attacks in Iraq )
The marketplace is filled with streets and alleyways that crisscross one another. The time of the attack -- about 4 p.m. (8 a.m. ET) -- is generally busy, with vendors selling electronics, produce, clothing and other items.
The arrested officer, whose name was not immediately released, was in command of the 2nd Brigade of the Iraqi army's 2nd Division, and will face an investigative committee looking into the bombings.
In all, bombings around the capital killed 198 people, the Interior Ministry reported.
In addition to the Sadriya attack, a suicide car bombing killed 35 people and wounded 55 near an Iraqi Army checkpoint at one of the entrances to Sadr City. Another bomb blast in central Baghdad's Karrada district killed 11 people and wounded 13, while bombs targeting police patrols in southern and southwestern Baghdad killed six police officers and two civilians, while wounding 14 people.
Another four civilians were killed when a bomb planted in a minibus detonated in Rusafi Square, a busy intersection in central Baghdad, the Iraqi Interior Ministry reported. Nine people were wounded.
The U.S. military reported four car bombs in Baghdad and issued a lower toll, saying 131 civilians are dead and around 164 are wounded. U.S. military counts are frequently lower than those supplied by Iraqi authorities.
The attacks occurred the same day British commanders transferred security control over the southern province of Maysan to Iraqi troops, a sign that Iraq is taking another step toward self-reliance. But the Baghdad blasts reflects the stiff challenges posed by what has been a resilient insurgency.
In Washington, Adm. William Fallon, the chief of U.S. forces in the Middle East, told a congressional committee that while he sees optimistic developments in Iraq, "there is hardly a week that goes by, certainly almost a day that doesn't go by without some major event that also causes us to lose ground."
Fallon told the House Armed Services Committee in Washington that the Baghdad security crackdown "is really the Iraqi leadership's major and potentially last opportunity to really take this ball forward."
"It's imperative, from my view, that they act on it," he said.
It is not clear whether Wednesday's attacks were linked, since many insurgent groups operate across Baghdad at any point in time. But most of the attacks targeted Shiite areas, and the market and checkpoint attacks, happened in areas where the Mehdi Army -- the militia of anti-American Shiite cleric Muqtada al-Sadr -- holds sway.
The Sadr City checkpoint bombing occurred at one of the new checkpoints set up for new crackdown in the capital.
Transfer of security in Maysan province
The Sadr City checkpoint bombing occurred at one of the new checkpoints set up in a new crackdown in the capital.
Meanwhile, the U.S. military was on the move against insurgents. Coalition raids in and around the Iraqi cities of Baghdad, Falluja, Taji, and Mosul on Wednesday led to the killing of six insurgents and the detention of more than 40 others, the U.S. military said.
A Task Force Marine soldier died in Baghdad on Tuesday of "non-battle injuries, the U.S. military said Wednesday.
Farther south in Maysan province, officials boasted that the transfer of security -- in what has been a relatively quiet district -- "demonstrates another step towards a stable and secure Iraq."
But Maysan isn't a sectarian flashpoint. It is a largely rural stretch located in the Shiite heartland in the south bordering Iran.
A statement from U.S. Ambassador to Iraq Ryan Crocker, British Ambassador Dominic Asquith, and U.S. Gen. David Petraeus said, "Maysan is the fourth of 18 provinces to be transitioned and this reflects Iraq's continued steps toward a capability to govern and protect itself and its citizens as a sovereign nation."
CNN's Jomana Karadsheh, Arwa Damon, Mohammed Tawfeeq, and Carolina Sanchez contributed to this report.