California's stand-off with US President Donald Trump's administration ratcheted up after a sitting US senator was on Thursday handcuffed and forcibly removed from a press conference on immigration raids that have spurred days of protests.

The shocking incident, which came after the Republican president sent troops into Los Angeles over the objections of local and state officials, was swiftly slammed by furious Democrats who said it "reeks of totalitarianism".

Video footage shows Senator Alex Padilla, a Democrat, being pushed from the room at a federal building in Los Angeles as he tried to ask Homeland Security Secretary Kristi Noem about the raids.

"I'm Senator Alex Padilla. I have questions for the secretary," he said as two men grappled with him in front of journalists.

Footage filmed by Padilla's staff outside the room shows the senator being pushed to the ground and handcuffed.

Democratic-led California is currently embroiled in battles with the White House on several fronts, with Governor Gavin Newsom branding Trump "dictatorial" as his lawyers prepared to face off with the administration over the deployment of 4,700 troops to the city.

"If this is how this administration 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," Padilla said later.

The incident "reeks of totalitarianism," Senate Minority Leader Chuck Schumer said, calling for an investigation.

"Trump and his shock troops are out of control," Newsom posted on social media, while Los Angeles mayor Karen Bass called the incident "abhorrent".

The White House hit back, claiming it was a "theater-kid stunt" and claiming without evidence that Padilla "lunged toward Secretary Noem".

Noem slammed Padilla's interruption as "inappropriate." A Homeland Security spokesman said she later met with the senator for 15 minutes.

Noem was addressing reporters after almost a week of protests in Los Angeles ignited by the Trump administration's immigration crackdown. (AFP)