Alex Padilla, Democrat of California, was shoved out of a room and handcuffed after he tried to question Kristi Noem, the homeland security secretary, during a news conference.

Senator Alex Padilla, Democrat of California, was forcibly removed on Thursday from a news conference being held by Kristi Noem, the homeland security secretary, and handcuffed after he interrupted Ms. Noem at a federal building in West Los Angeles.

“Sir! Sir! Hands off!” Mr. Padilla, 52, shouted as federal agents tried to muscle him out of the room inside a government office building about 15 miles west of downtown Los Angeles where Ms. Noem was speaking. “I am Senator Alex Padilla. I have a question for the secretary.”

As Mr. Padilla — an M.I.T. graduate, the son of Mexican immigrants and a Los Angeles native — began asking about a bank of mug shots behind Ms. Noem, agents shoved him out of the room, told him to drop to his knees in a hallway and handcuffed him, based on videos taken by Mr. Padilla’s office and a Fox News reporter.

A small group of reporters pivoted their cameras toward the disruption. Other national and local journalists were forced to wait outside the building after officials blocked access to the news conference shortly before the event began.

On the videos, Mr. Padilla appeared stunned but repeatedly said he was a U.S. senator. In an interview hours later, Mr. Padilla said that he had demanded to know why he had been detained and where he was being escorted “when of all people, Corey Lewandowski” — a combative former Trump campaign aide and adviser to Ms. Noem — “comes running down the hall and he starts yelling, ‘Let him go! Let him go!’”

In the tense hyperpartisanship of the moment, the episode quickly swelled into a cause célèbre for both parties. Democratic senators, House members and governors rushed to denounce the treatment of a sitting senator, framing it as the latest escalation in authoritarian actions by the Trump administration. It followed the indictment on Tuesday of Representative LaMonica McIver of New Jersey and the arrest of Mayor Ras Baraka of Newark, after the officials, both Democrats, tried to visit a new immigration detention facility in the city.

Republicans just as eagerly tried to frame Mr. Padilla’s behavior as in line with what they have called the lawlessness of the political left as President Trump tries to combat illegal immigration.

Ms. Noem had been at a lectern thanking the Army, Marines and National Guard for providing “security” when Mr. Padilla made his entrance. While some protests have erupted in downtown Los Angeles, the towering white federal building where her news conference occurred was more than 15 miles away from the action and had no protesters outside.

Mr. Padilla said in the interview on Thursday evening that he learned of Ms. Noem’s news conference while he was waiting for a scheduled briefing down the hall. He said he had asked for answers about the administration’s “increasingly extreme” immigration actions since January and had not been able to get them.

When he saw Ms. Noem and her entourage pass him, he said, he asked the National Guard member and an F.B.I. agent who had escorted him whether they would also take him into the news conference. They did, he said, and he initially stood at the back of the room, silently observing behind the cameras.

But when Ms. Noem said that federal agents were in Los Angeles “to liberate this city from the socialists and the burdensome leadership” of the Democratic leaders of California and Los Angeles, he conceded he could no longer stay silent.

The footage shows Mr. Padilla stepping to one side, introducing himself and starting to call out a question about mug shots that federal authorities had said were of violent undocumented criminals. He said he hoped to ask about others who have been detained with no criminal record, but, as he spoke, agents swarmed him and forcibly removed him from the room.

Ms. Noem and other Trump administration officials asserted that Mr. Padilla had failed to identify himself and had assumed a threatening demeanor.

“I will say that people need to identify themselves before they start lunging at people,” she said.

But Mr. Padilla said that he had not only identified himself in the room, as shown in the footage, but had also introduced himself to the agents who had escorted him from the lobby.

Tricia McLaughlin, a spokeswoman for the Department of Homeland Security, said that he had not been wearing his Senate security pin and that the Secret Service had taken him for an attacker. She accused Mr. Padilla of engaging in “disrespectful political theater.”

In a social media post, the F.B.I.’s deputy director, Dan Bongino, said that the bureau’s agents “acted completely appropriately while assisting Secret Service” agents and echoed the claim by other federal agencies that Mr. Padilla had physically resisted law enforcement and had failed to wear a security pin.

The F.B.I.’s chief spokesman, Ben Williamson, was unapologetic.

“When an unrecognized Senator in plain clothes and wearing no security pin became disruptive and subsequently resisted law enforcement, our F.B.I. L.A. personnel responded in support of Secret Service completely appropriately,” he wrote on social media. “We stand by them and appreciate their swift action.”

The Secret Service members involved in the encounter will not face discipline, said Anthony Guglielmi, a spokesman.

Afterward, Mr. Padilla and Ms. Noem met for about 15 minutes, both said. Mr. Padilla said that before Mr. Lewandowski instructed the agents to uncuff him, he had no idea whether he was about to be jailed or simply removed from the building. He said he had never been arrested or even handcuffed before.

Mr. Padilla said that the way he was treated as a sitting member of Congress had raised his level of concern for how the federal government has begun to treat everyday people.

“If this is how this administration responds to a senator with a question, if this is how the Department of Homeland Security responds to a senator with a question, you can only imagine what they’re doing to farm workers, to cooks, to day laborers out in the Los Angeles community and throughout California and throughout the country,” he said at a news conference, appearing to briefly be overcome with emotion.

The middle child of a short-order cook from Jalisco, Mexico, and a housekeeper from Chihuahua, Mr. Padilla was earning his way through school with janitorial jobs and work-study programs when his plan to become an aerospace engineer was derailed by the anti-immigrant politics that gripped California during the 1990s.

Galvanized by Proposition 187, a 1994 ballot initiative that would have barred undocumented immigrants from public services, including schools and nonemergency health care, he became involved in politics.

Though they represent the nation’s largest state in Congress’s high chamber, Mr. Padilla and the newly elected Adam Schiff still cut a low profile in the Senate. Thursday’s events could provide a political boost to Mr. Padilla, who was initially appointed to his seat by Gov. Gavin Newsom to replace Kamala Harris when she became vice president in 2021.

The episode stoked intense outrage in Los Angeles, where the ordinarily soft-spoken and slow-talking Mr. Padilla has long been a popular figure. On social media, the city’s mayor, Karen Bass, called his treatment “absolutely abhorrent and outrageous” and reiterated a demand she has made since a series of militarized immigration enforcement actions started in Southern California last Friday: “This administration’s violent attacks on our city must end.”

Mr. Newsom jumped in, calling the senator “one of the most decent people I know.”

“This is outrageous, dictatorial, and shameful,” Mr. Newsom wrote in a social media post. “Trump and his shock troops are out of control. This must end now.”

Senator Susan Collins, Republican of Maine, broke ranks with her party, calling what happened “very disturbing,” though she acknowledged she was unsure of what had led to the confrontation. “It looks like he is being manhandled and physically removed. It is hard to imagine a justification for that.”

Senator Lisa Murkowski of Alaska, another Republican moderate, called the incident “horrible” and “shocking at every level.” She continued: “It’s not the America I know.”

More in line with the mainstream Republican reaction was Representative Steve Scalise of Louisiana, the House majority leader, who told reporters in the Capitol that Mr. Padilla was in Los Angeles trying to “make the situation worse” and “stir angst against the federal agents who were coming to help” the city.

Senator Chuck Schumer of New York, the Democratic leader, addressed the Senate floor regarding the treatment of Mr. Padilla.

“I just saw something that sickened my stomach,” he said. “The manhandling of a United States senator. We need immediate answers to what the hell went on.”