Democratic US Senator Alex Padilla has been forcibly removed from a news conference held by Homeland Security Secretary Kristi Noem in Los Angeles.

Noem was offering the latest figures on immigration enforcement in the area, which has led to nearly a week of protests in the city, to members of the news media when Sen Padilla interrupted and started shouting a question.

Once removed from the room, the California senator was handcuffed.

Padilla's removal caused condemnation on both sides of the aisle, with fellow senators calling the arrest shocking and a "sickening disgrace" and the Trump administration dubbing it "disrespectful political theatre".

"I'm Senator Alex Padilla," he said as he was confronted by authorities. "I have questions for the secretary!"

Noem, who was speaking about immigration and the protests in LA, continued addressing reporters and law enforcement officers while the senator was ejected from the room.

Padilla's office said he was "forced to the ground and handcuffed" by federal agents when trying to ask the secretary a question, and added that he was not currently being detained.

The Department of Homeland Security said Padilla had engaged in "disrespectful political theatre" and that Noem met with the senator after the news briefing.

LA Mayor Karen Bass called the incident "absolutely abhorrent and outrageous", adding that the Trump administration's "violent attacks on our city must end".

Padilla told reporters that he was already in the federal building for a previously scheduled meeting. He said he stopped by Noem's news briefing because he and his colleagues have received "little to no information in response" to several immigration-related queries.

Padilla, the son of Mexican immigrants, is the most senior Democrat on the Senate's Border Security and Immigration subcommittee.

"I came to the press conference to hear what she had to say, to see if I could learn any new additional information," he said.

"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 labourers out in the Los Angeles community and throughout California and throughout the country."

He urged Americans across the country to "continue peacefully protesting" the Trump administration and its policies.

The DHS in a statement said Padilla didn't identify himself and was not wearing the Senator's pin on his clothing so officers thought he was an attacker.

Video footage of the incident shows Padilla saying he was he was a senator as he was being pushed outside the room.

California's Governor Gavin Newsom called on Republican congressional leadership to condemn the detaining of Padilla.

"If they can handcuff a U.S. Senator for asking a question, imagine what they will do to you," Newsom wrote on social media.

But the White House accused the California senator of storming the press conference, and said he "yelled and lunged toward Secretary Noem".

"Padilla didn't want answers; he wanted attention," Abigail Jackson, White House spokesperson said. "Padilla embarrassed himself and his constituents with this immature, theater-kid stunt – but it's telling that Democrats are more riled up about Padilla than they are about the violent riots and assaults on law enforcement in LA."

Former vice-president and Trump opponent, Kamala Harris also criticised the move and said the California senator was trying to get answers for his constituents about the ongoing immigration raids in the state.

"United States Senator Alex Padilla was representing the millions of Californians who are demanding answers to this Administration's actions in Southern California," she said on Twitter. "This is a shameful and stunning abuse of power."