BAGHDAD -- Five bombs exploded in and around Iraq's capital yesterday, killing at least 187 people and injuring more than 220 in the deadliest day since U.S. and Iraqi forces launched a much-publicized security crackdown two months ago.
Four of the five attacks targeted predominantly Shiite neighborhoods, sparking criticism of the security plan and heightening fears of revenge attacks and death-squad killings.
The bloodiest killed at least 127 people in Baghdad's Sadriya district, which was still recovering from a February car-bomb explosion that killed 137 people in a busy marketplace.
The attacks could be a setback for U.S. and Iraqi security forces, which announced Feb. 13 an aggressive plan to deploy thousands of additional U.S. soldiers and Iraqi police officers in and around Baghdad in an effort to restore security.
Under the plan, hundreds of traffic checkpoints sprouted overnight. U.S. troops increased raids against insurgent strongholds, arresting 3,000 people in two months. U.S. and Iraqi officials are establishing outposts in 10 city districts to bolster confidence.
But insurgents have demonstrated an equal determination to undermine the effort. Last week, insurgents blew up a key Baghdad bridge spanning the Tigris River, and a suicide bomber infiltrated heavy Green Zone security to detonate explosives inside parliament's cafeteria, killing a Sunni Arab lawmaker.
Victims of yesterday's late-afternoon attack in Sadriya included construction workers repairing buildings damaged in the previous bombing and rush-hour commuters lining up at a bus depot, waiting for rides home.
Those who survived questioned the effectiveness of the government campaign.
"What security plan?" asked Qassim Nadhum, 40, who sells frozen meat in Sadriya and was sprayed with shrapnel in the head and shoulder. "The violence is continuing. All we get is traffic jams."
Sattar Ali, 35, a bus driver who also was wounded, said the crackdown seemed to work at first, but insurgents appear to be pushing back harder.
"This is not a plan; it's just a show," he said. "It's a failure."
In addition to the casualties from the car-bomb attacks, police in the capital reported finding 17 bodies of people presumed executed in sectarian violence. The combined toll made yesterday one of the bloodiest days in the Baghdad area since the U.S. invasion in 2003.
Iraqi Prime Minister Nouri al-Maliki ordered the arrest and investigation of the army commander responsible for security in the Sadriya district, saying the two recent car bombings have exposed weaknesses in procedures to protect area citizens.
U.S. and Iraqi officials urged patience yesterday, insisting that the security program was working. But critics said the attacks demonstrated the futility of the Bush administration's policies in Iraq.
"We shouldn't expect immediate results," Shiite lawmaker Sahar Ata said. "We need to have time, at least a year."
Defense Secretary Robert M. Gates said he had warned that insurgents would attempt to increase the violence to derail the new plan. But he vowed to show that the security plan for Baghdad was not a failure.
"Today was a horrifying thing," Gates told reporters in Tel Aviv, Israel. "But I think it illustrates another point: These terrorists are killing innocent men, women and children who are Iraqis."
The bombings, Gates said, appeared to be an attempt to entice Shiite militias to respond with their own retaliatory killing.
"We can only hope that the Shia will have the confidence in their government and in the coalition, that we will go after the people that perpetrated this horror," he said.
Besides the market attack, bombs struck Shiite targets in the capital at a police checkpoint, near a hospital and in a small bus.
Nationwide the number of people killed or found dead was 233, which was second only to a total of 281 killed or found dead on Nov. 23, 2006. Those figures are according to Associated Press record-keeping, which began in May 2005.
Dana Perino, the White House spokeswoman, said such spectacular attacks, and higher death tolls among American troops, are to be expected as U.S. soldiers spread into strife-torn neighborhoods and insurgents redouble their efforts to counter increased troop numbers.
"Anybody who thinks that this enemy is tired, they are mistaken," Perino said. "They are a very determined enemy. They are watching what we are doing and what we are saying, and it's critically important that we finish the job in Iraq."
Many of the most devastating bomb attacks in the country have come in the past several months, indicating insurgents have developed more sophisticated or powerful explosives.
U.S. military officials announced that last week they found 3,000 gallons of nitric acid hidden in a warehouse in downtown Baghdad. U.S. forces discovered the acid, a key fertilizer component that can also be used in explosives, during a routine search April 12, the military said.
Timothy M. Swager, head of the chemistry department at the Massachusetts Institute of Technology, said that aside from being used to make explosives, nitric acid could cause dangerous burns if used directly on people.
Witnesses from the Sadriya bombing reported seeing a parked minibus explode, leaving a 15-foot-wide crater in the road and setting ablaze dozens of buses and taxis. Street vendors dumped their fruits and vegetables and used their wooden carts to evacuate victims.
"It was really chaos," said Abu Hussam, 51, who owns a paint shop nearby. "People are coming here to look for their loved ones."
Heavy afternoon traffic hindered rescue efforts and heightened the death toll, police said. Bodies lined the hallways of inundated hospitals.
Earlier in the afternoon, a suicide car bomb detonated at the eastern gateway to Sadr City, near a police checkpoint, the presumed target. At least 41 were killed.
During the noon hour, a parked car exploded near a private hospital in Karradah, a predominantly Shiite district in the center of Baghdad. At least 11 people died.
A suicide bomber struck a police patrol at nightfall in the Saydiyah neighborhood, a mixed Sunni-Shiite district in southwest Baghdad. Four died, including two policemen, and eight were wounded, five of them police, police officials said.
Edmund Sanders writes for the Los Angeles Times. The Associated Press contributed to this article.