President Trump fired six National Security Council officials after an extraordinary meeting in the Oval Office with the far-right activist Laura Loomer, who laid out a list of people she believed were disloyal to the president, U.S. officials said on Thursday.

The firings were described by one of the U.S. officials, who had direct knowledge of the matter. The decision came after Ms. Loomer vilified the staff members by name during a meeting on Wednesday, when she walked into the White House with a sheaf of papers attacking the character and loyalty of numerous N.S.C. officials. Michael Waltz, the national security adviser, joined later in the meeting and briefly defended some of his staff, though it was clear he had little if any power to protect their jobs.

It was a remarkable spectacle: Ms. Loomer, who has floated the baseless conspiracy theory that the Sept. 11 attacks were an “inside job” and is viewed as extreme by even some of Mr. Trump’s far-right allies, was apparently wielding more influence over the staff of the National Security Council than Mr. Waltz, who runs the agency. A longtime supporter of Mr. Trump who has frequently spoken of her desire to work with him, Ms. Loomer was one of Mr. Trump’s most vicious online enforcers during the 2024 campaign.

The account of the White House meeting with Ms. Loomer and the subsequent firings is based on interviews with eight people with knowledge of the events. They asked for anonymity to discuss confidential meetings and conversations.

The people fired included Brian Walsh, the senior director for intelligence; Maggie Dougherty, the senior director for international organizations; and Thomas Boodry, the senior director for legislative affairs. None could be reached for comment.

To some in the government, the firings felt arbitrary. Most if not all of the officials who have been targeted by Ms. Loomer were put through a personnel vetting process run by the Trump administration.