Immigration agents Wednesday detained 47 illegal aliens, some teen-agers, who apparently were living at a Woodbridge construction site in the first large local roundup of undocumented workers since 1984, Immigration and Naturalization Service officials said yesterday.

Some of the men, who were said to be working for subcontractors at the site, had been sleeping in the partly built apartments on blankets, sleeping bags and crushed cardboard boxes, immigration officials said.

Others were sleeping in a lean-to shanty made of plywood and in vans parked on the premises. The immigration agents found several doused campfires, indicating that the men also had been cooking there.

In the last six years, the Washington area has increasingly become a magnet for illegal immigrants fleeing economically ailing Mexico and strife-torn Central America, particularly El Salvador.

The dawn raid on the 30-acre apartment complex site on Davis Ford Road, under construction by Houston-based Blazer Building Inc., was the result of a two-month investigation into the transportation of illegal aliens to this area from other parts of the country, said Bradley Stevens, deputy director of the immigration service's Washington district office.

It was not related to the new immigration law that imposes fines of up to $10,000 and jail terms on employers who knowingly hire illegal workers, immigration officials said. Until June 1 of next year, the agency will issue only warnings to first offenders and fine repeat offenders. No warnings have as yet been issued, immigration officials said.

Most of the 47 were from Mexico, a few were from El Salvador, and seven were under the age of 18, said John Wright, an immigration service supervisory special agent who participated in the raid.

Officials said they will arrange transportation in the next several days for the illegal workers who have decided to return home voluntarily. Those who choose to appeal their cases will remain in custody while awaiting deportation hearings.

The immigration service arrested and charged four men -- two of them illegal aliens and two foreign-born immigrants who have permission to work in this country -- with illegally transporting and harboring undocumented workers, a federal crime that carries a fine of up to $2,000 and up to five years in jail per alien.

Two of the men, one who has applied for amnesty under the new legalization program and the other a legal resident of this country, were released on their own recognizance yesterday. The two illegal aliens are being held without bond pending a preliminary hearing before a federal magistrate.

The men allegedly brought the illegal workers from Texas, Ohio and North Carolina, Wright said.

Steve Wice, a Blazer employe and the superintendent of the Woodbridge construction site, said the illegal workers were not employed by Blazer but by subcontractors. "I really don't have any employes to worry about because we are in construction management," Wice said. "The subcontractors are the ones who have to meet the requirements {of the new immigration law} and make sure their people are legal."

Wice also said he doubted the illegal workers had been sleeping at the job site. "It's our policy not to let people sleep on the job site," he said.

The new immigration law says businesses that employ subcontractors are not responsible for verifying that the subcontractors hire legal workers. However, the law says that businesses cannot use subcontractors to circumvent the law and hire illegal workers.