US Senator Alex Padilla of California was forcibly removed and handcuffed by federal agents on Thursday after interrupting a news conference by Homeland Security Secretary Kristi Noem in downtown Los Angeles.

Video footage of the altercation, which quickly went viral, shows Padilla, a Democrat, the first Latino to represent California in the US Senate, identifying himself as a senator before being physically subdued.

Federal officials claimed Padilla lunged toward Noem and refused repeated commands to step back.

Supporters said the senator was trying to speak out against the deployment of troops for immigration raids across California.

Padilla said later on MSNBC that his removal was "excessive" and "an overreaction". He explained that he had been escorted into the briefing room by federal agents while waiting for a scheduled meeting with US Northern Command.

"I stood in the back, listening, until the political rhetoric got to be too much to take," he said. "So I spoke up."

Also on Thursday, a US District Court judge in San Francisco ruled that the federalization of parts of California's National Guard by Trump was "illegal" and ordered him to return control of the Guard to Governor Gavin Newsom.

California sued the federal government over its deployment of National Guard troops in Los Angeles for immigration enforcement.

Judge Charles Breyer sharply questioned the legal basis for the Trump administration's actions.

The White House has argued that the president's decision to mobilize troops could not be second-guessed by the courts.

"That's not where we live," Breyer said in court. "We live in response to a monarch. This country was founded in response to a monarch. The Constitution is a document of limitations."

Breyer expressed concern over the Trump administration's decision to bypass Newsom in activating the state's National Guard, noting that federal law requires gubernatorial consent in such deployments.

Meanwhile, on Capitol Hill, Democratic lawmakers condemned the treatment of Padilla, calling it an abuse of federal power. Senate Minority Leader Chuck Schumer, a New York Democrat, said on the Senate floor: "I just saw something that sickened my stomach, the manhandling of a United States senator. We need immediate answers to what the hell went on."

House Speaker Mike Johnson, a Republican, sided with law enforcement and suggested the Senate should discipline Padilla.

Secretary Noem later said she had spoken with Padilla and called his behavior "inappropriate", though she did not elaborate.

Former vice-president Kamala Harris, Padilla's predecessor in the Senate, issued a statement calling his removal "a shameful and stunning abuse of power".

She added, "Senator Padilla was representing the millions of Californians who are demanding answers to this administration's actions in Southern California."

Newsom condemned the incident on social media, calling it "outrageous, dictatorial, and shameful. If they can handcuff a US senator for asking a question, imagine what they will do to you."

California Attorney General Rob Bonta described the federal agents' actions as "shameful and disrespectful," adding, "Using force to silence (Padilla) is unacceptable."

Los Angeles Mayor Karen Bass called Padilla's removal "absolutely abhorrent and outrageous. … This administration's violent attacks on our city must end."

Los Angeles County Supervisor Janice Hahn said: "This administration is out of control. Senator Padilla is just as much a part of the federal government as they are."

Last week, border czar Tom Homan threatened to arrest and prosecute Newsom and Bass if they interfered with Immigration and Customs Enforcement (ICE) operations — a warning that was later endorsed by the president. Homan later said there was "no intention" to arrest the governor.

Authorities have now arrested more than 1,000 people nationwide in connection with protests that erupted in response to workplace immigration raids in Los Angeles last week.

What began as local demonstrations in LA has since spread to cities across the country. While many protests have remained peaceful, others have escalated into confrontations marked by vandalism, property damage and clashes that left both demonstrators and law enforcement officers injured.

As legal and political battles play out, protests are expected to further intensify on Saturday, when major demonstrations have been planned to coincide with President Trump's 79th birthday and a military parade in Washington DC.

Agencies contributed to the story.