U.S. Capitol Police were aware of the possibility of violence from armed extremists when the Capitol was attacked by supporters of former President Donald Trump, but quickly became overwhelmed by "insurrectionists", the acting police chief said.

Intelligence collected ahead of the Jan. 6 assault on the Capitol suggested that the gathering would have the participation of armed "militia members, white supremacists, and other extremist groups," Yogananda Pittman said in a prepared testimony to be delivered Thursday before the House Appropriations Committee.

Pittman was the assistant chief of police of the department's Protective and Intelligence Operations on Jan. 6 and said she was responsible for its Intelligence and Interagency Coordination Division (IICD).

Trump supporters saw Jan. 6 as the last opportunity to overturn the results of the presidential election and their sense of desperation and disappointment "may lead to more of an incentive to become violent," the IICD said in an assessment issued on Jan. 3, according to Pittman.

"While the Department was prepared to neutralize and remove individuals or groups engaging in civil disobedience or violence among the demonstrators, it was quickly overwhelmed by the thousands of insurrectionists (many armed) who immediately and without provocation began attacking officers, bypassing physical barriers, and refusing to comply with lawful orders," Pittman added.

Supporters of Trump stormed the Capitol building in an attempt to stop Congress from certifying Democratic President Joe Biden's electoral victory over Republican Trump, who falsely claimed that the November election had been marred by widespread fraud. 

The Capitol attack delayed the certification of Biden's win by several hours, as lawmakers were forced to flee to seek safety from the angry mob that overwhelmed security forces. Five people died in the violence.

More than 200 people have been charged so far for their roles in the riot, including some with ties to far-right fringe groups such as the Oath Keepers and Proud Boys.

An FBI warning that a protest by Trump supporters could turn violent reached the U.S. Capitol Police the day before the assault, but top officials in charge of securing Congress that day did not see it, they told lawmakers on Tuesday.

(Reporting by Kanishka Singh in Bengaluru; Editing by Nick Macfie) ((Kanishka.Singh@thomsonreuters.com; +91 8061822801;))