Fairfax County police recently held a Spanish-language child safety seat demonstration in Herndon, hoping it would build goodwill between the department and the area's burgeoning Latino population.

Instead, it underscored the mistrust caused by a new state law that allows local police to detain some illegal immigrants: No one from the immigrant community showed up.

According to Capt. Mike Vencak of the Reston District station, which planned the safety demonstration, some in the Hispanic community "thought it was a ploy" to snare the undocumented. Elsewhere in Virginia, the law has prompted immigrants to hoard food and stay indoors, police and community leaders said. One Latina in Fairfax, a victim of domestic violence, would not go to the police for help, fearing she would be deported, an immigrant advocate said.

A little over a week after it took effect, the law -- sometimes called "HB570" after its bill number -- is fracturing the already fragile trust that bridged the law enforcement and immigrant communities, critics of the legislation said. It was intended to be limited in scope, but it is inciting a much broader reaction of fear and mistrust of the police, they said.

"This is the kind of law that makes a vulnerable community even more vulnerable," said Arlington County Supervisor Walter Tejada (D), who has been holding forums to educate Latinos on the law. "Immigrant communities are already reluctant to contact the police if they are victims of a crime or a witness to a crime. Now it will make the communities even more hesitant."

The law, which went into effect July 1, is a landmark of sorts in Virginia. For the first time, state and local authorities can arrest illegal immigrants without a warrant, but only if they have been convicted of a felony, already have been ordered out of the country and are suspected of committing another crime.

Primary sponsor Del. David B. Albo (R-Fairfax) said only a few people meet all three criteria -- but they could be society's most dangerous criminals.

Several police officers testified during legislative hearings that they worked to convict gang members of felonies and to have them deported, only to see them back in their neighborhoods.

Now they'll have another tool to arrest them, Albo said.

"I bet if you talk to immigrants in this country, 99 percent of them will say we don't want those people in our neighborhoods," he said.

Albo added that he believes a few civic leaders, not the law, are at the root of the immigrant community's fear. "There's a bunch of people from the community who are developing a hysteria that I don't understand," he said. "I don't know why they are doing it."

But immigrant advocates counter that the fear is very real. Many immigrants have a natural distrust of the police because they come from countries where the authorities are corrupt, said Tim Freilich, managing attorney of the Virginia Justice Center, an advocacy group.

"The amount of damage that the passage of this law has already caused between the police and immigrant communities far outweighs any potential benefit," he said.

Scot Christenson, media relations coordinator for the Catholic Legal Immigration Network, said the law could lead to racial profiling and hurt police departments if it discourages immigrants from reporting crimes.

"It is a security issue," he said. "Police groups can't solve crimes or investigate if people won't come forward, and what they are basically doing is alienating a huge chunk of the population."

The number of immigrants, drawn to Virginia by an abundance of jobs in such fields as construction, is increasing rapidly in the state. Spanish-speakers are the most numerous among immigrants and composed 11 percent, or 214,000 people, of the total population of Northern Virginia in 2000, according to the U.S. Census.

The law's passage has been a daily topic on media outlets within the immigrant enclaves. Sgt. Richard Perez, a Fairfax police spokesman, said he has appeared on Spanish-language television and radio stations to explain the new law. Each time, his office has been flooded with calls as soon as he left the air.

Hispanic residents, he said, "are under the impression that this law was going to be used to start deporting all illegal aliens." Perez added that "people are contemplating hoarding food and staying inside to withstand the initial onslaught of enforcement."

Despite efforts to educate immigrants on the law's limitations, mistrust of the police is spreading, immigrant advocates said. The fear might not be rational, they said, but it is a natural reaction.

Part of the problem is that it can be hard to prove one's status, said Frecia Guzman, a Salvadorian immigrant who owns a deli in Fairfax.

She noted that many of her Salvadorian friends are in the United States with a special asylum grant known as Temporary Protected Status. But many don't have the papers to prove it and have been waiting for months to get their documents from the federal government.

Now they are afraid of venturing outside their homes, believing they will get stopped by police and asked to produce documents that prove their immigration status, she said.

"They are scared to go out to the restaurants," Guzman said. "They don't even want to go outside."

Arlington Supervisor Walter Tejada has been holding forums on the law, which, he said, "makes a vulnerable community even more vulnerable." Many immigrants distrust police, said Virginia Justice Center's Tim Freilich.