President-elect Joe Biden ended his first speech following the Electoral College vote with an appeal to all Americans to unite as the coronavirus continues to spread.

He acknowledged the latest grim milestone in the pandemic, 300,000 Americans killed by the virus.

"My heart goes out to each of you in this dark winter of the pandemic, about to spend the holidays and the new year with a black hole in your hearts, without the ones you loved at your side," he said.

Just as he did during his speech on Nov. 7 when he was declared the winner of the presidential election, Biden invoked his Catholic faith in his call for comfort and cited the prayer of St. Francis.

"For where there is discord, union. Where there is doubt, faith. Where there is darkness, light," Biden said.

Biden reiterated that he will be a "president for all Americans" and called on everyone to "lower the temperature" and come together.

"We're a great nation. We're good people. We may come from different places, hold different beliefs, but we share in common a love for this country, a belief in its limitless possibilities," he said. "For we, the United States of America, has always set the example for the world, for a peaceful transition of power."

President-elect Joe Biden addressed the nation, just minutes after the Electoral College declared him the victor of the 2020 election and reflected on the long road it took to get to this stage.

Biden, who was awarded 306 electoral votes, commended the American public, particularly election workers and officials, for running the election during the deadliest pandemic in recent history.

"The flame of democracy was lit in this nation a long time ago, and we now know-nothing, not even a pandemic or an abuse of power can extinguish that flame," he said during the nearly 15-minute speech.

Biden noted that his Electoral College tally equaled President Donald Trump's 2016 victory.

"By his own standards, these numbers represented a clear victory then, and I respectfully suggest they do so now," he said.

Biden had harsher words for the president and some of his GOP supporters who backed his weeks of legal challenges to overturn the results of the election. He slammed the failed lawsuit issued by Texas' attorney general to invalidate votes in swing states.

"It's a position so extreme we've never seen it before, a position that refused to respect the rule of the people, refused to respect the rule of law and refused to honor our Constitution. Thankfully a unanimous Supreme Court immediately and completely rejected this effort," he said.

"The courts sent a clear signal to President Trump that they would be no part of an unprecedented assault on our democracy."

Moments after Biden secured his Electoral College victory, Trump tweeted that Attorney General Bill Barr has resigned.

"Just had a very nice meeting with Attorney General Bill Barr at the White House. Our relationship has been a very good one, he has done an outstanding job! As per letter, Bill will be leaving just before Christmas to spend the holidays with his family," Trump said in a tweet, attaching Barr's resignation letter in a thread.

The resignation comes after Trump and Barr's relationship deteriorated in the weeks following the election. Barr, earlier this month, directly undercut allegations being made by Trump and his legal team when he said the Justice Department had uncovered no evidence of widespread voter fraud that would tip the results of the presidential election.

Barr's letter praises Trump for his leadership, casts him as a victim of "partisan onslaught" and "relentless" resistance and echoes some of his most scathing criticism targeting the Russia investigation. He makes no mention of the tensions that have recently flared between the two, and even seeks to play to Trump on the way out the door, noting that their meeting was on voter fraud in the 2020 election and allegations he says are still being pursued.

Trump also announced on Twitter that Deputy Attorney General Jeff Rosen will become Acting Attorney General -- for the final five weeks of his presidency.

-ABC News' Alexander Mallin

California, a crucial vote-rich state, has officially sealed Joe Biden's victory, with the state's 55 electors delivering the president-elect the majority in the Electoral College.

By the current count, Biden now has 302 electoral votes, compared to Trump's 232.

Presiding over California's meeting as chair was Assembly Member, Shirley Weber, who announced that all 55 electors cast their votes for Biden. The room stood to clap and celebrate his victory in the state.

It's significant that it's California, Vice President-elect Kamala Harris' home state, that handed Biden his formal victory in the Electoral College -- and another blow to Trump's effort to overturn the election.

-ABC News' Kendall Karson