A new preventive detention law increased the number of federal prisoners by 32 percent last year, putting fresh strains on an already overcrowded prison system, Justice Department officials said yesterday.

Stanley E. Morris, director of the U.S. Marshals Service, said that most of the 2,000 additional prisoners each month were criminal defendants held without bail under the Comprehensive Crime Control Act. The law, which took effect in October 1984, makes it far easier for judges to detain defendants without bail if they are deemed dangerous or likely to flee before trial.

"One could make a very good case that this is having an impact on crime, that people who shouldn't be on the street are not on the street," Morris told a news conference.

While it is too early for definitive conclusions, Morris said, the new law has enabled the government to detain drug dealers, organized crime figures and other cash-rich defendants who could meet almost any level of bail. More convicted felons also are being jailed pending appeal, he said.

But Morris also acknowledged that the influx of prisoners -- from a monthly average of 5,608 in fiscal 1984 to 7,426 last year -- has further overloaded a prison system "that's been in crisis for a long time and remains in crisis.

"We don't have adequate jails in the United States to respond to the public's demand to do something about crime . . . . You put more people in prison and jails and you exacerbate the problem," he said.

The Marshals Service, which has custody of prisoners before and during trial, said in a report yesterday that lengthy pretrial detention hearings spawned by the new law have increased the average prisoner's stay by 12 percent. In one case, pretrial hearings for a Puerto Rican gang charged with a $7 million robbery took five weeks and cost the service $309,000.

Critics say the Justice Department has been seeking preventive detention in numerous cases in which defendants should be entitled to bail. "It's a terrible thing for the Constitution," said Charles Sims, a lawyer for the American Civil Liberties Union. "We think it's being used far too broadly."

Morris said federal judges have approved such no-bail requests 82 percent of the time. But the Marshals Service has been hard-pressed to find cells for the growing number of detainees, despite the biggest prison construction program in federal history.

"It will get worse before it gets better," Morris said.

The federal system, which now houses 36,700 inmates, is 42 percent over its capacity. In 1980, the system was 2 percent under capacity with 23,780 prisoners. Moreover, many of the state and local jails that the Marshals Service uses for detainees are under court order to ease overcrowding.

In Los Angeles, for example, the Terminal Island federal prison has 1,050 inmates and detainees, which is more than double its design capacity. Officials were forced to double-cell all inmates when the overcrowded local jails in Los Angeles stopped accepting federal prisoners.

"The tension is there," said Richard Rison, warden of the 47-year-old prison. "We are feeling the strain that goes along with overcrowded conditions . . . . It's like putting four or five families in a single-family house."

The 1984 law also made it easier for the government to seize assets of criminal defendants upon arrest. The Marshals Service now owns $313 million in cash and property, including ranches, resorts, banks, farms, restaurants and condominiums.

"We own one of the finest rock recording studios in Sausalito, Calif.," Morris said, referring to a facility that was seized from drug dealers.

Seizure of assets increased 40 percent in the first year after the law took effect, with the most lucrative seizures, involving real property, doubling to 260. Such assets must be returned if the defendant is later acquitted.

Here too, Morris said, the new law has not been without problems. He said the government must search for management firms to run these enterprises and has "made some mistakes" in its choice of investments.

In one case, federal marshals seized Western State Bank in Denton, Tex., which was part of a drug dealer's $60 million empire. But the bank turned out to have so many bad loans that its liabilities exceeded its assets, and it was later returned to local authorities.