WASHINGTON — Supporters of President Donald Trump stormed the US Capitol on Wednesday as Congress began a joint session underway to confirm Joe Biden as the country's next president.

Senators were being evacuated, while lawmakers tweeted they were sheltering in place in their offices.

Demonstrators fought with Capitol Police and then forced their way into the building.

Clashes followed a massive rally near the White House during which Trump egged them on to march to Capitol Hill.

But as tensions mounted at the capitol, Trump tweeted to supporters to "stay peaceful".

Lawmakers had convened for an extraordinary joint session to confirm the Electoral College results but protests erupted outside the Capitol and government office buildings were being evacuated.

The session started at 13:00 local time (19:00 CET) but was suspended 15 minutes later after objections over the results in Arizona — the third of the country's 50 states to announce its Electoral College count — were raised.

The session was expected to last into the night as Republican House members have said they would object to as many as six battleground states. Lawmakers from the House and Senate must debate each objection.

Outgoing President Donald Trump has alleged widespread fraud in the Nov. 3 election but has failed to produce any evidence to support his claims. Republicans have attempted to overturn the results in several key states through the courts but been have been unsuccessful.

Democrat Biden, 78, won the Electoral College 306-232 and is to be inaugurated on Jan. 20.

As the session started, Trump held a mass rally in Washington in which he vowed to "never concede" and falsely claimed his Vice President, Mike Pence, has the power to delay the confirmation by sending Electoral College votes back to the states to be recertified.

“Mike Pence is going to have to come through for us," Trump said, "and if he doesn’t it’s a sad day for our country."

He also criticized members of his party who have not supported his attempt to overturn the results as "weak".

Pence, however, refuted Trump's claim in a letter.

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

The session to confirm Biden as the 46th president of the United States comes as the Democrats look poised to flip the Senate on Wednesday as the results of the run-off election in Georgia come in.

Raphael Warnock, a 51-year-old pastor from Atlanta, is projected to have unseated Republican Senator Kelly Loeffler, while former documentary filmmaker Jon Ossof, 33, currently has a narrow lead over incumbent David Perdue.

If both Democrat contenders win their races, the balance in the Senate would be 50-50 which would make Vice President-elect Kamala Harris the tie-breaker to pass laws. With the lower house already held by Democrats, Joe Biden's ability to push through his agenda would be massively strengthened.

However, if one of the two Republican incumbents in Georgia retains their seats, they would then be able to block court nominees and legislation.— Euronews