A coalition of advocates and community leaders yesterday encouraged immigrants to continue to cooperate with local police departments on criminal investigations despite a new Virginia law allowing them to detain some illegal immigrants without a warrant.

The coalition, the Virginia Alliance for Sensible Community Police Efforts, announced at a news conference that it would monitor police departments to guard against racial profiling and other abuses.

"We are in favor of stopping terrorism, gang activity and drug trafficking, but what this law will do is make a vulnerable community" even more vulnerable, said J. Walter Tejada, an Arlington County Board member.

Tejada said misinformation about the law has caused fear among immigrants. A possible consequence, he said, is that immigrants "will not report crime."

The law, which will take effect July 1, allows police to arrest and hold illegal immigrants who return to this country after being convicted of felonies and deported. Police will be able to hold suspects as long as 72 hours without bond while waiting for federal immigration agents to pick them up.

It is one of several steps that Virginia officials have taken to tighten security after the Sept. 11, 2001, terrorist attacks.

Local law enforcement officials have sought to reassure immigrant communities by meeting with advocates and holding community meetings. They also have been conducting training sessions for senior police officials on how the law should be applied, said Dana Schrad, executive director of the Virginia Association of Chiefs of Police.

"I don't think there will be a problem," Schrad said. "There are clear-cut parameters when this particular authority will be used by local and state police. This is not an opportunity to go out and round up a bunch of illegal residents."

However, Jorge E. Figueredo, executive director of the Hispanic Committee of Virginia, said he would encourage immigrants to call the committee if they see abuses. His group will establish a database to help monitor detentions, he said.

Another member of the new coalition, the Virginia Justice Center, will join with the American Civil Liberties Union and the Mexican American Legal Defense and Educational Fund to monitor how police agencies apply the law, said Tim Freilich, managing attorney at the center.

Schrad said the law would not dramatically change the relationships between police and immigrant communities. She called the law a "procedural change" that "will allow a little more information sharing between local, state and federal officials who investigate these cases."

"If you are a law-abiding citizen, you are not the intended subject of this statute," Schrad said. "The intent of this statute is to enable Virginia law enforcement agencies to work with federal law enforcement officials to address terrorism and violent gang activity that may involve illegal residents."

Tim Freilich of the Virginia Justice Center speaks at a news conference on the new Virginia law.