ASHINGTON, June 6 — More than 13,000 of the Arab and Muslim men who came forward earlier this year to register with immigration authorities — roughly 16 percent of the total — may now face deportation, government officials say.

Only a handful have been linked to terrorism. But of the 82,000 men older than 16 who registered, more than 13,000 have been found to be living in this country illegally, officials say.

Many had hoped to win leniency by demonstrating their willingness to cooperate with the campaign against terror. The men were not promised special treatment, however, and officials believe that most will be expelled in what is likely to be the largest wave of deportations after the Sept. 11 attacks.

The government has initiated deportation proceedings, and in immigrant communities across the country, an exodus has already begun.

Quietly, the fabric of neighborhoods is thinning. Families are packing up; some are splitting up. Rather than come forward and risk deportation, an unknowable number of immigrants have burrowed deeper underground. Others have simply left — for Canada or for their homeland.

The deportations are a striking example of how the Bush administration increasingly uses the nation's immigration system as a weapon in the battle against terror.

For decades, illegal immigrants have often flourished because officials lacked the staff, resources and political will to deport them. But since the attacks on the World Trade Center and the Pentagon, the government has been detaining and deporting illegal immigrants from countries considered breeding grounds for terrorists.

"There's been a major shift in our priorities," said Jim Chaparro, acting director for interior enforcement at the Department of Homeland Security, which has subsumed the old immigration service.

"We need to focus our enforcement efforts on the biggest threats," Mr. Chaparro added. "If a loophole can be exploited by an immigrant, it can also be exploited by a terrorist."

Advocates for immigrants warn that such a strategy — indeed, the administration's sweeping reorientation of law enforcement toward terrorism prevention — can be abused by government officials.

They note that, though it did not deal directly with the registration program, an internal Justice Department report was released this week that was deeply critical of the government's roundup of illegal immigrants after Sept. 11, 2001. Senior officials were found to have repeatedly ignored calls from immigration officials to quickly distinguish between the innocent and guilty. As a result, many people who had no ties to terrorism were jailed unnecessarily, the report said.

Advocates for immigrants have also accused officials of practicing selective enforcement by focusing on illegal immigrants from Arab and Muslim nations. Rather than disrupting communities, they say, the government should improve its intelligence and prosecution of terrorists.

"What the government is doing is very aggressively targeting particular nationalities for enforcement of immigration law," said Lucas Guttentag, director of the immigrants' rights project at the American Civil Liberties Union. "The identical violation committed by, say, a Mexican immigrant is not enforced in the same way."

Some of those facing deportation have waited months or years for officials to process applications to legalize their status. Immigration lawyers say they believe that a substantial number of these men avoid deportation. Their clients are only illegal, the lawyers say, because of the government's inefficiency.

Even before the registration program began early this year, the government had deported hundreds of illegal immigrants in its effort to prevent, and not simply respond to, terrorist attacks. But the scope of those deportations remains unclear.

Officials say more than 600 Arab and Muslim illegal immigrants were deported during the first wave of expulsions after Sept. 11. But the Justice Department stopped releasing figures after the number of arrested immigrants surged to 1,200, and officials have declined to give complete statistics for that period.

Another wave of deportations began last year after officials said they planned to find and arrest illegal immigrants who pose security threats and already have deportation orders. Of that group, more than 3,000 people have been arrested. Officials say they cannot say how many of those Arab and Muslim men have been deported.

But it is the special registration program — which required noncitizens from 25 Arab and Muslim countries to register from December through April — that seems likely to produce the largest number of expulsions. In the last two months, officials have released a succession of tallies of immigrants facing deportation; the 13,000 figure represents the most up-to-date estimate.

Officials acknowledged that most Arab and Muslim immigrants swept up in counterterrorism sweeps have no ties to terrorist groups. Of the 82,000 men who showed up at immigration offices, and tens of thousands more screened at airports and border crossings in the past six months, 11 have had links to terrorism.

Still, officials said, they can no longer ignore illegal immigrants from countries that pose a security risk. They noted that several Sept. 11 hijackers were in the country illegally at the time of the attacks.

In all, deportations of illegal immigrants from Asian and African countries have surged by nearly 27 percent in the last two years. The number of Pakistani, Jordanians, Lebanese and Moroccans deported during that time has doubled, the statistics show; the number of Egyptians deported has nearly tripled.

In the immigrant communities, the impact multiplies.

The Pakistani Embassy in Washington said that since Sept. 11, more than 15,000 Pakistanis in this country illegally are believed to have left for Canada, Europe and Pakistan.

Three weeks ago, in Stanton, Calif., Yasser Tair, a Syrian real estate agent, was deported to the country he left 14 years ago. Mr. Tair's wife, Zhour, and his two American-born children are also moving.

Their dining room table is already gone. So are the rugs, the children's beds, the computer. "I am selling everything," Zhour Tair said. "I cannot make it here anymore."

In Brooklyn, hundreds of illegal immigrants have sought help from the Arab-American Family Support Center. About 500 of the center's clients who registered are now fighting deportation in court.

Johanna Habib, the center's only staff lawyer, writes their names and court dates on an increasingly crowded wall calendar. Some of the men have American-born children or wives who are citizens or green-card holders. Many have applications to legalize their immigration status that have been pending for years.

"We have a handful that are going to be O.K." Ms. Habib said. "But it's a very small handful."

Abdel Hakim Benbader, a cabdriver from Algeria at the center, said he registered even though he was living here illegally, because he wanted to help. He said he thought his future was in the United States. His wife has a green card and his son is an American citizen.

"We were thinking of buying a house," said Mr. Benbader, who is fighting deportation. "We were thinking of having good jobs like everybody. If I'm leaving, it's going to be a big problem."