Police and protesters clashed outside the Democratic party's national headquarters on Wednesday, forcing dozens of lawmakers to be evacuated.
Officials said the pro-Palestinian organisers calling for a ceasefire in Gaza were gathered "illegally and violently".
Demonstrators said they were provoked by police in riot gear who rushed them without warning.
Six officers were treated for injuries and a protester was arrested.
Officers said they were pepper sprayed and punched by people in the crowd.
The incident unfolded on the front steps of the Democratic National Committee headquarters in Washington DC, where political leaders, including the highest-ranking House Democrat, had gathered for a reception.
Chants of "ceasefire now" from roughly 150 activists outside the event resounded in the halls.
Capitol Police swiftly descended on the crowd to block off the building's entrance and exit.
Demonstrators said the policing quickly turned aggressive.
Video footage of the incident appears to show protesters shouting and screaming as the police barrel in to restrain them.
"It is shameful the way that nonviolent protesters and members of our community were met with violence tonight," Dani Noble, a demonstrator who arrived from Philadelphia, told the Washington Post.
He said police were "pulling on folks that are disabled or have chronic illnesses, pulling people to the ground".
Inside, police rushed to direct lawmakers to safety. Security has been tightened around the building since pipe bombs were found at both the Democratic and Republican Party headquarters on the eve of the 6 January 2021 Capitol riot. No-one has ever been arrested for the planting of those devices.
California congressman Brad Sherman wrote on X, formerly Twitter: "Was just evacuated from the #DNC after... protestors grew violent, pepper spraying police officers and attempting to break into the building."
In a statement shared with US media, House Democratic leaders said members of the House Democratic Caucus, aides and guests were participating in an event inside when "some protesters escalated their activity in a manner that exceeded a peaceful demonstration".
"We are thankful for the service and professionalism of the US Capitol Police officers who worked to ensure that Members, staff and visitors were able to safely exit," the leaders said.
"We strongly support the First Amendment right to freedom of expression and encourage anyone exercising that right to do so peacefully."
The protest was organised by a coalition of groups, including Jewish Voice for Peace Action and If Not Now, both US Jewish groups who are critical of Israel.
The confrontation on Wednesday was the latest in a series of demonstrations across the US, as tensions over the Israel-Gaza war intensify.
On Tuesday, thousands of pro-Israel protesters crowded the fields at the National Mall in Washington DC, demanding the release of Hamas-held hostages.
A number of rallies have pressed for the US to call for a ceasefire in the conflict, which President Joe Biden, a Democrat, has resisted.
On Capitol Hill, many Republicans have rejected the idea of a ceasefire outright.
The militant group Hamas entered Israel on 7 October and killed roughly 1,200 people while kidnapping more than 200 others. In response, Israel has killed more than 11,000 people in Gaza, according to the Hamas-run healthy ministry.