About 300 Muslim community leaders from around the United States attended the dinner. Having a United States attorney general speak at such an event was unprecedented, the group’s president, Farhana Khera, said in an interview earlier on Friday.

Mr. Holder was given a standing ovation as he took the stage, and many applauded during his speech. But the room fell silent for several minutes while Mr. Holder defended the sting operation in an Oregon bombing attempt last month, calling it a “successful undercover operation” and not a case of entrapment. Those who think otherwise, he said, “simply do not have their facts straight.”

Ms. Khera said the group had invited the attorney general several months ago. She portrayed the Muslim-American community as torn by a mistrust of law enforcement because of what it sees as intrusive surveillance and harassment — like sending informants into mosques — and by its concerns about anti-Muslim hate crimes.

Mr. Holder said the cooperation of Muslim-Americans had been essential in preventing terrorist attacks. He said that the Justice Department was focusing on “violence, threats, vandalism and arson against Muslims and Arab-Americans,” and that in the last fiscal year federal prosecutors won convictions of more hate-crime defendants than any year but 2000.

“I believe that law enforcement has an obligation to ensure that members of every religious community enjoy the ability to worship and to practice their faith in peace, free from intimidation, violence or suspicion,” Mr. Holder said.

But he also rejected criticism of some counterterrorism techniques used by the Federal Bureau of Investigation, including sending informants into mosques in search of would-be terrorists and creating elaborate sting operations enabling them to carry out fake attacks using dummy bombs.

Ms. Khera emphasized that Muslim Advocates recognized that “there are actual threats that do exist and as Americans who care about the country, we want law enforcement to be effective.”

But the complex “entrapment operations,” she contended, may be getting people involved in terrorism who otherwise would not have done anything. She also argued that the operations divert investigators from “actual threats” and stoke “anti-Muslim sentiment.”

At a reception after the speech, many in the audience voiced their gratitude for Mr. Holder’s presence, saying it would help rebuild trust between U.S. law enforcement and Muslims. “This is a positive step toward engaging a vital community and perhaps one of the most important partners in combating extremism and terrorism in America,” said Wajahat Ali, 30, a lawyer and playwright from Fremont, Calif. “He said exactly what needed to be said. Now those words need to be translated into action.”

In his remarks, Mr. Holder said that stings had been used for decades against many types of crimes. And he defended the investigation last month in Portland, Ore., in which a young Somali-American man, Mohamed Osman Mohamud, 19, was arrested after law enforcement agents said he tried to trigger what he thought was a car bomb at a Christmas tree-lighting ceremony.

“I make no apologies for the how the F.B.I. agents handled their work in executing the operation that led to Mr. Mohamud’s arrest,” Mr. Holder said. “Their efforts helped to identify a person who repeatedly expressed his desire and intention to kill innocent Americans.”

He added: “But you also have my word that the Justice Department will — just as vigorously — continue to pursue anyone who would target Muslims, or their houses of worship.”

Despite the attorney general’s reassurances, some in attendance were deeply concerned by the federal government’s ongoing undercover sting operations.

“I grew up during the civil rights era and I’m aware how the civil rights community was infiltrated by provocateurs and agents who sought to undermine the legitimate struggles of the movement,” said Abu Qadir Al-Amin, 60, an African-American imam from Vallejo, Calif. “So my antennae are up and I try to educate the Muslim community so that they don’t put themselves in a vulnerable position if someone comes along suggesting they do something illegal.”