Parts of Capitol Hill are evacuated as protesters flood the grounds.

The Capitol Police ordered the evacuation of at least one office building on Capitol Hill as protesters flooded the grounds as part of a widespread protest against the certification of President-elect Joseph R. Biden Jr.’s victory.

Police officers ran door to door in the Cannon House Office Building ordering the evacuation before the alert reached some offices, according to one person working in the building. While the alert sent to Capitol offices cited only “police activity,” videos posted on social media showed hundreds of protesters barreling past fence barricades outside the Capitol and clashing with officers.

Representative Nancy Mace, a freshman Republican from South Carolina, described seeing protesters “assaulting Capitol Police.” In a Twitter post, Ms. Mace shared a video of the chaos and wrote, “This is wrong. This is not who we are. I’m heartbroken for our nation today.”

The police fired what appeared to be flash-bang grenades. Rather than disperse, the protesters cheered and shouted, “push forward, push forward.” One protester shouted, “that’s our house,” meaning the Capitol

As officers and protesters clashed outside, lawmakers were debating an objection to the certification of Arizona electors, ensconced in their respective chambers. Senator Mitch McConnell, Republican of Kentucky and the majority leader, warned of a “death spiral” for democracy, while Representative Jim Jordan, Republican of Ohio, listed a litany of accusations of election fraud with little evidence.

“I don’t recognize our country today, and the members of Congress who have supported this anarchy do not deserve to represent their fellow Americans,” said Representative Elaine Luria, Democrat of Virginia.

Trump supporters were seen trying to tear down security barriers, confronting the police and trying to enter the building.