Trump supporters were seen scaling the Capitol building steps as the proceedings continued inside. Some breached security to make it inside, to near the Senate chamber.

Senate Minority Leader Chuck Schumer lectured lawmakers in the chamber, saying an element of the GOP "believes their political viability hinges on the endorsement of an attempted coup."

"By the end of the proceedings today it will be confirmed once again something that is well-known and well-settled: the American people elected Joe Biden and Kamala Harris to be the next president and vice president of the United States," Schumer said.

Schumer said Republicans know this fact but will object to the counting of the vote anyway -- "and in the process they will embarrass themselves, they will embarrass their party, and worst of all they will embarrass their country."

"As we speak, half of our voters are being conditioned by the outgoing president to believe that when his party loses an election, the results must not be legitimate. As we speak the eyes of the world are on this chamber questioning whether America’s still the shining example of democracy, the shining city on the hills. What message we send today, what message will we send today to our people, to the world that has so looked up to us for centuries?”

As Schumer spoke, protesters breached the perimeter outside the U.S. Capitol Building and the building went on lockdown.

A notice from United States Capitol Police to all buildings within the Capitol Complex said, "Due to an external security threat located on the West Front of the U.S. Capitol Building, no entry or exit is permitted at this time. You may move throughout the building(s) but stay away from exterior windows and doors. If you are outside, seek cover."

-ABC News' Mariam Khan, Allie Pecorin and Trish Turner

Kicking off up to two hours of debate on the Senate floor after some GOP lawmakers objected to the electoral vote count in Arizona, Senate Majority Leader Mitch McConnell made an extraordinary speech rebuking his fellow Republicans for their actions.

"We’re debating a step that has never been taken in American history – whether Congress should overrule the voters and overturn a presidential election."

Speaking directly to his colleagues, he reminded them he supported Trump's right to legal challenges, "But over and over, the courts rejected these claims including all-star judges whom the president himself has nominated."

"We cannot simply declare ourselves a national board of elections on steroids. The voters, the courts, and the states have all spoken,” he said. “If we overrule them, it would damage our republic forever."

“This election is not unusually close,” he added.

McConnell went on to say overturning the results would send the country on a "poisonous path” and into an "endless spiral of partisan vengeance” before calling on elected leaders to summon "patriotic courage" in both victory and in defeat.

"We must not imitate and escalate what we repudiate," he said.

"I will not pretend such a vote would be a harmless protest gesture while relying on others to do the right thing. I will vote to respect the people's decision and defend our system of government as we know it," McConnell said in closing.

-ABC News' Trish Turner

As expected, the first election results have been challenged in Congress with GOP Sen. Ted Cruz of Texas and Rep. Paul Gosar of Arizona objecting to the counting of Arizona's Electoral College votes.

"Are there any objections to counting the certificate of vote of the state of Arizona that the teller has verified periods to be regular in form and authentic?" said Vice President Mike Pence, presiding over the session, as he will for each state.

"I write for both myself and 60 of my colleagues to object to the counting of the electoral ballots from Arizona," Gosar said.

"Is the objection in writing and signed by a senator?" said Pence.

"Yes, it is," Cruz proudly said, to applause from GOP lawmakers, with some rising to their feet.

Senators will go into the Senate and House members will stay in the House for two hours of debate on the certification of Arizona’s vote.

Minutes before the session began, Pence was heard asking an aide if his statement was out -- in which said he does not believe it's in his power to object the Electoral College count -- and the aide said yes.

"Thank you," Pence said.

Earlier, House Speaker Nancy Pelosi admonished Republicans for gathering on their side of the chamber and their lack of social distancing, given the guidance issued asking members to stay in their offices and not congregate on the House floor unless they are voting.

-ABC News' Mariam Khan and Benjamin Siegel

The joint session of Congress to confirm the Electoral College vote won by Biden is underway.

What's usually a short, ceremonial event is expected to turn into a marathon affair. However, the end result will be the same -- Biden will still be the next president, set to be inaugurated in just two weeks.

Vice President Pence is the presiding officer and in an uncomfortable situation as most legal experts agree he does not have the constitutional authority to object to Electoral College vote counting yet President Trump -- down Pennsylvania Avenue at a "Save America" rally -- demanded Pence "have the courage" to intervene, Trump saying he will be "very disappointed." if he doesn't.

Moments before the session began, Pence released a statement through the White House making clear he does not believe he has the authority to intervene in the election

"It is my considered judgment that my oath to support and defend the Constitution contains me from claiming unilateral authority to determine which electoral votes should be counted and which should not," Pence said.

The Vice President's job is to go through ceremonial mahogany boxes full of the votes from the states, ticking off each one alphabetically -- but this year, it won't be long until a senator objects.

When Pence gets to Arizona, Republican Sen. Ted Cruz, a Trump loyalist, is expected to object, thus launching up to two hours of debate in each chamber. Thirteen senators and as many as 140 House Republicans have said they will make objections in the process, but it's still unclear how many states will be challenged. Republican Sen. Josh Hawley of Missouri has said he'll object regarding Pennsylvania. Hawley has also said he does not believe Pence has the ability to not accept the electoral certification.

-ABC News' Trish Turner