Insurgents seeking to topple Iraq's interim government are targeting the young men who gather in long lines to seek jobs in the country's nascent security services, killing scores of them in a campaign that poses a direct threat to U.S. efforts to increase the size of Iraq's police and National Guard.

In the latest attack on aspiring recruits, a suicide bombing on Tuesday, a car bomb laden with artillery shells tore through a crowd of men who had been forced to stand outside blast-absorbing concrete barriers at a police headquarters in central Baghdad. At least 47 people were killed and 114 wounded, police and public health officials said.

More than 700 policemen and prospective recruits have been killed in insurgent attacks since U.S. forces toppled Saddam Hussein's government in April 2003, according to Iraq's Interior Ministry. Most were killed in suicide bombings, and many were simply applying for jobs.

The applicants are particularly vulnerable because they are often forced to wait outside the concrete barriers set up to protect police stations from car bombs. Police officials say they generally refuse to allow job seekers freely through the perimeter because of concern that some may be suicide attackers.

In July, 70 people were killed when a car bomb exploded outside a police recruiting center in Baqubah, northwest of the capital. In February, more than 100 recruits and civilians were killed in suicide attacks at two recruiting stations in Baghdad and Iskandariyah, about 30 miles south.

The attacks, which appear intended to dissuade people from applying for jobs in the security services, have created a challenge for the United States and the U.S.-backed interim government as they seek to significantly increase the size and quality of Iraq's police force and National Guard. On Tuesday, the Bush administration asked Congress for authority to use part of an $18.4 billion reconstruction aid package to train and equip 45,000 more police officers -- a 50 percent increase in the size of the force -- and 14,000 more National Guardsmen.

So far, however, the attacks have not stopped young Iraqi men from seeking jobs as police officers. Those who show up at recruiting centers typically say they are motivated not by patriotism but by economics. Many maintain that they have no choice but to assume the risk because there are few other well-paying jobs in postwar Iraq.

"There are no other jobs," said a police officer at the scene of Tuesday's bombing who gave his name as Hussein. "Joining the police and the army is the only choice."

Ali Yassen, an unemployed 20-year-old who had been waiting near the site of the blast, said it was the promise of a paycheck that had led him to apply to be a policeman, widely viewed as the most risky profession in Iraq today. "I hated this job before I even started it," he said.

U.S. and Iraqi officials involved in the recruitment and training of security forces have expressed concern that the violence has led many veteran officers and experienced recruits to seek jobs with private security firms, leaving the force with a pool of less-qualified candidates.

"Do we have to find ways to make it safer to apply to be a police officer?" one senior American official said. "Of course we do. But what do you do? Do you bring them in the perimeter? Do you install hundreds of new walls to protect them?"

U.S. intelligence officials say they believe that insurgent leaders have assigned suicide car bombers to drive around Baghdad with orders to target whatever concentrations of Westerners or Iraqi security forces they find.

Shortly after Tuesday's bombing, gunmen ambushed a bus ferrying policemen in Baqubah, killing 11 officers and one civilian. Later in the day, a second suicide car bomb detonated in downtown Baghdad as a convoy of Western contractors passed, but only the driver was killed in the mistimed blast, according to witnesses.

A militant group loyal to Abu Musab Zarqawi, a Jordanian linked to al Qaeda who is one of most wanted men in Iraq, asserted responsibility for the police headquarters bombing, which ripped through a bustling intersection. The blast punched an eight-foot-wide crater into the roadway and pulverized many victims. When rescue workers arrived, they found disoriented survivors screaming for help amid piles of disfigured bodies.

"They were after those poor young men who wanted to join the Iraqi police," Hussein said. "I can't understand why Iraqis kill other Iraqis."

The police building is just off Haifa Street, where numerous clashes between U.S. forces and insurgents have occurred in the past few weeks.

"We knew that something like this would happen, because a large group of young men gather here every day," said Abu Ali, the owner of a small shop near the scene.

Although Zarqawi's group asserted responsibility, many bystanders and relatives at the scene were quick to assign blame, both direct and indirect, to the United States. Some blamed the presence of U.S. troops in Iraq for provoking attacks on Iraqis working with the interim government.

More than 150 Iraqis have been killed in insurgent attacks and clashes with U.S. forces since Sunday. In the western city of Ramadi, at least eight civilians were killed and 18 were wounded on Tuesday in clashes between U.S. troops and insurgents, according to medical officials quoted by the Associated Press.

The U.S. military announced that a U.S. soldier was killed and five were wounded when their patrol was attacked with small arms fire in the northern city of Mosul on Tuesday. Separately, the military said that two soldiers were killed and three were wounded Monday in an attack involving a roadside bomb and small arms fire.

In other developments Tuesday, officials reported an attack on oil pipelines crossing the Tigris River in northern Iraq. The sabotage knocked out one of Iraq's largest power plants, shutting off electricity to much of Baghdad for part of the day.

Firefighters struggled to put out the blaze after the attack 155 miles north of Baghdad, near Baiji, news services said. U.S. military officials surveying the blast estimated it could take as long as three days to put out the fire.

Special correspondents Naseer Nouri and Bassam Sebti contributed to this report.

Iraqis gather at the scene of a massive suicide car bombing outside a police headquarters in central Baghdad that killed at least 47 people and wounded 114.