Election 2020 updates: Biden warns of 'dark winter,' pushes masks in pandemic plan

From New York to Philadelphia to Washington, D.C., to Atlanta to Los Angeles, Americans are cheering, honking horns and taking to the streets to celebrate Biden's win.

In Washington, D.C., revelers at Black Lives Matter Plaza popped champagne bottles and sang “Celebrate Good Times."

Patrisse Cullors, co-founder and executive director of Black Lives Matter Global Network Foundation, said in a statement, "Black voters showed up in huge numbers to turn this country around and remove the racist in the White House. What is abundantly clear is Black voters were the factor that tipped the scales in favor of Joe Biden and Kamala Harris, especially in Rust Belt battleground states. It's striking that the same people who have been treated the worst by our democracy are doing the most to save it."

"We congratulate Joe Biden on becoming President, and particularly Kamala Harris, on becoming the country’s first woman - a Black woman - to serve as Vice President. This historic win is a testament to the work Black women have been doing in the streets, in this campaign, and at every level of politics," Cullors said.

Senate minority leader Chuck Schumer celebrated on the streets of New York, proclaiming that the "long dark night in America" is over.

"So I say to Donald Trump: You lost. No more games. Go home. Go home to Florida," Schumer said. "Stop making up lies about the election. It was fair. There have been no irregularities found."

Schumer also encouraged Democrats to get involved in the Georgia Senate runoff race this January.

"Tell anyone you know in Georgia make sure you vote," Schumer said.

ABC News projects Biden will win the battleground state of Nevada.

Former President Barack Obama "spoke separately this afternoon with president-elect Joe Biden and vice president-elect Kamala Harris," an Obama spokesperson said. "He congratulated them on their historic victory and told them how proud he was of the campaign they ran in unprecedented times.”

Obama said in a statement Saturday that he "couldn't be prouder" of Biden and Harris.

"We’re fortunate that Joe’s got what it takes to be President and already carries himself that way. Because when he walks into the White House in January, he’ll face a series of extraordinary challenges no incoming President ever has – a raging pandemic, an unequal economy and justice system, a democracy at risk, and a climate in peril," Obama said.

"I know he’ll do the job with the best interests of every American at heart, whether or not he had their vote. So I encourage every American to give him a chance and lend him your support. The election results at every level show that the country remains deeply and bitterly divided," he said.

"Our democracy needs all of us more than ever," Obama said. "And Michelle and I look forward to supporting our next President and First Lady however we can."

Former President Jimmy Carter said in a statement, "Rosalynn joins me in congratulating our friends President-elect Joe Biden and Vice President-elect Kamala Harris. We are proud of their well-run campaign and look forward to seeing the positive change they bring to our nation."

Former President Bill Clinton tweeted, "America has spoken and democracy has won. Now we have a President-Elect and Vice President-Elect who will serve all of us and bring us all together. Congratulations to Joe Biden and Kamala Harris on your momentous victory!

Trump’s lawyer, Rudy Giuliani, on Saturday vowed to file a lawsuit Monday to challenge the conduct of elections officials in Pennsylvania.

In comments laced with vague and unfounded allegations of voter fraud in Pennsylvania, Giuliani alleged that the Trump campaign was deprived of the ability to watch the ballots being processed. And he said the campaign would make similar allegations in other states that could lead the campaign to make the case a “massive nation-wide lawsuit.”

“We were deprived of the right to inspect if a single one of those ballots is legitimate,” Giuliani said. “That is unheard of, it’s illegal, it’s unconstitutional, and we will be bringing an action challenging that.”

The Trump campaign has now brought multiple similar charges into court since Election Day and all but one of the cases has been rejected – in Michigan and Nevada. In Pennsylvania, a state judge ordered election officials to allow observers to move closer to poll workers, but a federal judge refused to halt the count based on the claims.

In several of the cases, judges have used strikingly similar language to criticize Trump campaign attorneys for bringing the cases without sufficient evidence. Biden attorney Bob Bauer on Thursday called the lawsuits "meritless."

Observers are permitted to watch the processing of ballots in most states, including Pennsylvania, but not allowed to intervene as the ballots are processed. They can note any irregularities and report them back to their campaign’s legal team.

Giuliani alleged that Trump campaign representatives were not close enough to see if ballots had errors or irregularities. But for days, Trump campaign officials have not been able to explain what remedy they could expect should a court agree that they were not given sufficient access to watch the poll workers open envelopes of mail-in ballots and inspect them to make sure signatures match, dates are accurate, and the secrecy envelope is used properly.

-ABC News' Matt Mosk and Alex Hosenball