Rep. Jerrold Nadler, delivering opening remarks at the hearing, said Barr's tenure as the nation's top law enforcement officer "has been marked by a persistent war" against the Justice Department's "professional core in an apparent attempt to secure favors for the president."
"In your time at the department, you have aided and abetted the worst failings of the president," he added.
Barr appeared before the panel to testify about his leadership of the department and answer criticisms on issues such as politicizing the department and the federal police response to anti-racism protests.
The hearing was originally scheduled to start at 10 a.m., but was briefly delayed after Nadler was involved in a traffic collision. He was unhurt.
Nadler criticized Barr for sending federal police to Portland, Ore., to monitor demonstrations outside the U.S. courthouse -- action that was taken over the objections of state and local officials in Oregon. The clashes have resulted in a number of arrests.
"There is no precedent for the Department of Justice actively seeking out conflict with American citizens, under such flimsy pretext, or for such petty purposes," Nadler said.
Rep. Jim Jordan, the panel's ranking Republican, defended Barr in his opening statement and played a 10-minute video clip of images that showed violent protests.
Democrats have accused Barr of politicizing the Justice Department and using its authority to protect Trump rather than the American people.
Democratic lawmakers, including Nadler, sent a letter last week to inspectors general at the departments of Justice and Homeland Security requesting an investigation into reports that Trump administration officials have abused emergency authorities to prevent Americans from exercising their right to peaceful assembly.
In his opening remarks, Barr said some of the Portland demonstrators have been overly violent and protests have damaged federal property.
"What unfolds nightly around the courthouse cannot reasonably be called a protest; it is, by any objective measure, an assault on the government of the United States," he said.
"Rioters have barricaded the front door of the courthouse, pried plywood off the windows with crowbars, and thrown commercial-grade fireworks into the building in an apparent attempt to burn it down with federal personnel inside. The rioters have started fires outside the building, and then systematically attacked federal law enforcement officers who attempt to put them out -- for example, by pelting the officers with rocks, frozen water bottles, cans of food, and balloons filled with fecal matter.
"Remarkably, the response from many in the media and local elected offices to this organized assault has been to blame the federal government. To state what should be obvious ... such acts are in fact federal crimes under statutes enacted by this Congress."
Barr also testified that Democrats have sought to "discredit" him over the department's Russia investigation, which he called a "bogus" scandal.
Barr was criticized after receiving the report from special counsel Robert Mueller in early 2019, particularly for refusing to publicize the entire report -- and instead releasing only a four-page summary that largely excluded Mueller's most serious criticisms of Trump.
Barr said an attorney general has a "unique obligation" to ensure that the standard of justice is applied to everyone equally, and that he's sought to uphold this obligation without interference from Trump.
"The president has not attempted to interfere in these decisions," he said. "On the contrary, he has told me from the start that he expects me to exercise my independent judgment to make whatever call I think is right. That is precisely what I have done."
Barr said that he did not discuss his recommendation for a lighter sentence for Trump ally Roger Stone with him, anyone at the White House or outside the Justice Department.
"Do you think it's fair for a 67-year-old man to be sent to prison for seven to nine years?" Barr said.
Stone was sentenced to 40 months in prison after prosecutors, who recommended a sentence of seven years to nine years, were overruled by Barr, prompting four of the prosecutors in the case quit, two of whom resigned entirely from the U.S. Attorney's Office in Washington, D.C. Trump commuted the sentence earlier this month.
Barr also did not commit to withholding the results of U.S. Attorney John Durham's investigation into the origin of the FBI's Russia investigation until after the 2020 presidential election, saying any report released ahead of the election "will be in my judgment not the one that is covered" by a Justice Department policy against investigations that may disrupt an election.
"We're not going to interfere," he said. "In fact, I've made it very clear that I'm not going to tolerate it."
Further discussing November's elections, Barr said there was no reason to believe the election would be "rigged" but did back Trump's skepticism of mail-in voting, saying "there's a high risk" for voter fraud if the practice is widely adopted.