WASHINGTON, Jan. 7 (Xinhua) -- With White House officials resigning in protest at the chaos and violence that overran the Capitol by supporters of President Donald Trump, the outgoing leader on Thursday issued a call for unity and promised an orderly transfer of power.

A large crowd of Trump's supporters forced their way into the Capitol on Wednesday afternoon when Congress met to formally count the Electoral College votes and affirm Biden's win.

The chaotic and disruptive situation, described by police as the "greatest breach of the U.S. Capitol since the War of 1812," forced an hours-long lockdown on the historic building complex and left four people dead, with multiple injuries and dozens of arrests. Thousands of individuals were involved in "violent riotous actions," according to Capitol Police.

Stephanie Grisham, chief of staff for U.S. first lady Melania Trump, and National Security Council Senior Director for Asian Affairs Matt Pottinger submitted their resignations Wednesday in the wake of the unprecedented scene at the Capitol, U.S. news outlets quoted officials as saying.

They were followed on Thursday by at least five senior directors at the White House National Security Council responsible for advising Trump on Russia, the Middle East, Africa, defense policy and weapons of mass destruction, the outlets said.

Also on Thursday, U.S. Transportation Secretary Elaine Chao and Education Secretary Betsy DeVos resigned.

In a letter to her colleagues at the Department of Transportation, Chao said her resignation will take effect on Monday.

"Yesterday, our country experienced a traumatic and entirely avoidable event as supporters of the President stormed the Capitol building following a rally he addressed," she wrote. "As I'm sure is the case with many of you, it has deeply troubled me in a way that I simply cannot set aside."

"There is no mistaking the impact your rhetoric had on the situation, and it is the inflection point for me," DeVos wrote in a letter dated on Thursday to Trump.

Trump's initial reaction to the incidents at the Capitol fell short of condemning the violence but doubled down on unsubstantiated allegations that the presidential election was "fraudulent," despite dozens of losses in courts over attempts to overturn the outcome. He encouraged his supporters to stage demonstration on Capitol Hill during a rally outside the White House on Wednesday noon.

Top congressional Democrats are calling for efforts to immediately initiate the removal of Trump from office.

During a press conference in the Capitol on Thursday, House Speaker Nancy Pelosi said Democrats may impeach the Republican for the second time if Vice President Mike Pence and the presidential cabinet fail to invoke the 25th Amendment, which outlines the transition of power if the president is unable or unfit to serve.

"What happened at the U.S. Capitol yesterday was an insurrection against the United States, incited by President Trump," Senate Democratic Leader Chuck Schumer tweeted on Thursday. "This president must not hold office one day longer."

Senator Lindsey Graham, a South Carolina Republican and Trump ally, warned that the president's legacy had been undercut by the Capitol violence but stressed that he wasn't supportive "at this point" of ousting Trump with the 25th Amendment.

Trump has reportedly been asking aides and lawyers, including White House counsel Pat Cipollone, about his self-pardon power.

"We have just been through an intense election and emotions are high. But now tempers must be cooled and calm restored," Trump said in taped remarks posted on Twitter on Thursday. "This moment calls for healing and reconciliation."

The Republican said he is outraged by "the violence, lawlessness, and mayhem" and vowed consequences for those "broke the law," while acknowledging the congressional certification of President-elect and Democrat Joe Biden's victory in their White House race in 2020.

"Now Congress has certified the results. A new administration will be inaugurated on January 20. My focus now turns to ensuring a smooth, orderly, and seamless transition of power," he noted.

A spokesperson for Biden's presidential transition team said in a statement on Thursday that Biden and Vice President-elect Kamala Harris are "focused on their duty" -- the transition work in preparation for their inauguration -- "and will leave it to Vice President Pence, the Cabinet and the Congress to act as they see fit."

The U.S. Secret Service, which is responsible for designing and implementing operational security plans for so-called National Security Special Events, said on Thursday that the safety and security of all those participating in Biden's inauguration is "of the utmost importance."

The agency said it has been working with relevant partners for over a year to "anticipate and prepare for all possible contingencies at every level to ensure a safe and secure Inauguration Day."