WASHINGTON (AP) – Protesters stormed the U.S. Capitol on Wednesday in a chaotic demonstration forcing lawmakers to be rushed from the building.

The ordinarily mundane procedure of certifying a new president was always going to be extraordinary, with some supporters of President Donald Trump vowing to protest results of an election that they have insisted was reversed by fraud. President-elect Joe Biden is to be inaugurated January 20.

The deliberations inside were still in their early stages when they were overcome by demonstrations outside, as protesters who clashed with police entered the building, shouting and waving posters and American flags. They abruptly interrupted the proceedings in a scene that featured eerie official warnings directing people to duck under their seats for cover and put on gas masks.

With the crowds showing no signs of abating, Mr. Trump tweeted, “Please support our Capitol Police and Law Enforcement. They are truly on the side of our Country. Stay peaceful!”

Senators were being evacuated. Some House lawmakers tweeted they were sheltering in place in their offices.

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

Lawmakers had convened for an extraordinary joint session to confirm the Electoral College results.

Though some Republicans were behind the challenge to Mr. Biden’s 306-232 Electoral College victory, Senate Majority Leader Mitch McConnell sought to lower tensions and argued against it. He warned the country “cannot keep drifting apart into two separate tribes” with “separate facts.”

Mr. McConnell declared, “The voters, the courts and the states all have spoken.”

Vice President Mike Pence was closely watched as he stepped onto the dais to preside over the joint session in the House chamber.

Mr. Pence has a largely ceremonial role, opening the sealed envelopes from the states after they are carried in mahogany boxes used for the occasion, and reading the results aloud.

In a statement shortly before presiding, Mr. Pence said he could not claim “unilateral authority” to reject the electoral votes that make Mr. Biden president.