The often personal and passionate stories made a striking contrast with the often dry, bureaucratic testimony more frequently heard on Capitol Hill.

“Our children are in danger,” said Melvin Bledsoe of Memphis, whose son, Carlos, is accused of attacking a military recruiting center in Little Rock, Ark., in 2009, killing one soldier and wounding another.

Mr. Bledsoe, one of two witnesses to testify about how a family member had embraced a violent brand of Islam, described radical Islam as “a big elephant in the room” and recounted how Islamic radicals in Yemen “trained and programmed” his son “to kill.”

The more than four hours of testimony on Thursday began with the opening statement of Representative Peter T. King, the New York Republican and chairman of the House Homeland Security Committee. Mr. King fiercely defended the hearings — lambasted for days by critics as a revival of McCarthyism — by vowing to continue with his inquiry, saying that “to back down would be a craven surrender to political correctness” and that there was “nothing radical or un-American” in the hearings.

“Indeed, Congressional investigation of Muslim American radicalization is the logical response to the repeated and urgent warnings which the Obama administration has been making in recent months,” he said.

Mr. King quoted top Obama administration officials as attesting to the threat of homegrown terrorism, listing a half-dozen American citizens and residents accused of plotting or carrying out violence in the name of Islam since 2009.

But the committee’s top Democrat, Representative Bennie Thompson of Mississippi, said Muslims as a community should not be accused of radicalism or violence. He suggested that such accusations could play into the hands of Al Qaeda by supporting its claim that the United States is hostile to Islam.

“I cannot help but wonder whether this hearing, focused on the Muslim American community, will be used to inspire a new generation of suicide bombers,” Mr. Thompson said. Noting his own roots in Mississippi, he also referred to Mr. King’s Irish heritage; some critics of Mr. King have noted that he was a strong supporter of the Irish Republican Army when it carried out terrorist attacks in the 1980s.

Mr. Thompson noted the arrest in the case of a bomb planted near a Martin Luther King Day march in Spokane, Wash., of a man reportedly associated with a white supremacist group. He said he hoped the committee would hold another hearing on the threat from anti-government extremists and white supremacists.

Another Democrat, Representative Keith Ellison of Minnesota, who is Muslim, choked up as he recounted the case of Mohammed Salman Hamdani, a 23-year-old volunteer medical technician who died in the collapse of the World Trade Center on Sept. 11, 2001, after rushing to help at the scene of the attacks. His disappearance and his religion led to false suspicions that he might have been part of the plot until his remains were identified.

The hearing is the first in a series that Mr. King says will explore the threat of Islamic fundamentalism inside the United States. The session, titled “The Extent of Radicalization in the American Muslim Community and That Community’s Response” was also to examine what the congressman asserts is the failure of some Muslims to cooperate with law enforcement.

Mr. King and another witness, Representative Frank R. Wolf, Republican of Virginia, singled out one national group, the Council on American Islamic Relations, or CAIR, for sharp criticism as an ally of radicalism and an opponent of cooperation with law enforcement. In singling out CAIR, Mr. King pointed to a placard that he said was distributed by the group in California. “Build a Wall of Resistance,” it declared. “Don’t Talk to the F.B.I.”

Controversy over the hearings had been building for weeks, and Mr. King had been under intense pressure from critics to either delay or broaden the hearing’s scope. Many Muslims fear they will be made targets, while religious and civil rights leaders are protesting what they see as ethnic profiling and the singling out of a particular minority.

Because counterterrorism officials rely on the cooperation of Muslims for tips and to foil plots, some law enforcement authorities are also raising alarms. They are concerned that the sessions will have the opposite of their intended effect, by making Muslims, who may already be nervous about talking to the authorities, even more nervous about doing so.

To drive home that point, Mr. Thompson invited Sheriff Leroy Baca of Los Angeles County, who has deep ties with the Muslim community there, to be the Democrats’ lead witness.

In his testimony, Sheriff Baca contradicted Mr. King’s premise that Muslim Americans do not cooperate with terrorism investigations, saying that was not his experience. He cited a study finding more violent extremist plots in the United States since 2001 by non-Muslims than by Muslims and that seven of the last 10 terrorist plots were foiled with the help of tips from Muslims.

“We should be examining radicalization as a problem that affects all groups,” he said.

In his testimony, Mr. Bledsoe of Memphis described how his only son, Carlos, grew estranged from the family after converting to Islam in college, shocking his parents by removing a portrait of Martin Luther King Jr. from his bedroom wall. He later traveled to Yemen, where he came into contact with Islamic radicals.

“Our society continues not to see it,” Mr. Bledsoe said of his concern of radical Islam.

Another witness, Abdirizak Bihi, who runs the Somali Education and Social Advocacy Center in Minneapolis, described how his nephew was among a group of young Somali Americans lured from Minnesota to Somalia to join an Islamic extremist group, Al Shabab. While there, the boy was killed.

The only representative of Muslim advocacy groups was Dr. M. Zuhdi Jasser, a physician from Arizona who formed the American Islamic Forum for Democracy to oppose what he saw as the failure of more prominent groups to openly fight radicalism.

“We can call everybody a bigot or an Islamophobe if they even talk about it, but radicalization is a problem that’s increasing exponentially,” Dr. Jasser said, adding that his parents had emigrated from Syria and believed they could worship as Muslims in the United State more freely than anywhere else in the world.

Dr. Jasser called for a “counter-jihad,” arguing that many American mosques promote “political Islam” rather than “spiritual Islam,” in part because they were founded using money from Saudi Arabia, which promotes a fundamentalist strain of the religion. While much of the hearing shows the polarization of the political debate that had preceded it, there were moments of overlap. Sheriff Baca, for instance, acknowledged the extreme views of some Muslim Americans, describing a young man who approached him at a mosque and admonished him for holding a Koran, suggesting it was prohibited for non-Muslims.

And Mr. King, who has stated in the past that 85 percent of leaders of American mosques hold extremist views, said he believed most Muslims in this country were “outstanding Americans.”

Representative John D. Dingell, Democrat of Michigan, whose district includes large Muslim populations, also testified.