By IVAN PERIERA, ABC News

(WASHINGTON) — Supporters of President Donald Trump who rallied outside the U.S. Capitol clashed with police officers and breached the building, forcing a lockdown with members of Congress inside.

A law enforcement source told ABC News shots were fired. It is unknown if there are any injuries. Images show officers with weapons drawn.

The Federal Protective Service and Secret Service are en route to assist the police, according to a source.

The clashes began as Trump and his allies held a rally pushing the Senate to not certify the election for President-elect Joe Biden.

The protesters pushed ahead of barricades and a wall of police officers in riot gear to get into the building. Inside, the Trump supporters, many of whom were not seen wearing face coverings, waved “Trump 2020” flags, shouted chants and moved freely.

“If you are in a public space, find a place to hide or seek cover,” U.S. Capitol Police told people inside the building.

According to reports, one of the protesters was in the dais of the Senate chamber and some were going door to door demanding “Where the f— are they?” They were also banging on the doors, according to reports.

President Trump, who was at the White House, tweeted at 2:39 p.m., “Please support our Capitol Police and Law Enforcement. They are truly on the side of our Country. Stay peaceful!”

Fifteen minutes earlier, he tweeted that Vice President Mike Pence “didn’t have the courage to do what should have been done to protect our Country and our Constitution,” and “USA demands the truth!”

During the rally, Trump said he would not concede and called on the supporters to march up to the Capitol. He promised the crowd he would be with him, but did not follow-up his promise and went back to the White House.

U.S. Sen. Mitt Romney told reporters who were with him in secure position, “This is what the president has caused today, this insurrection.” Romney had been accosted by a Trump supporter at an airport Tuesday.

U.S. Rep. Jim Himes, D-Conn., tweeted, “Police have asked us to get gas masks out as there has been tear gas used in the rotunda.”

Washington, D.C., Mayor Muriel Bowser issued a citywide curfew beginning at 6 p.m. and ending Thursday at 6 a.m.

“During the hours of the curfew, no person, other than persons designated by the Mayor, shall walk, bike, run, loiter, stand, or motor by car or other mode of transport upon any street, alley, park, or other public place within the District,” the mayor’s office said.

ABC News’ Jack Date and Luke Barr contributed to this report

This is a developing story. Please check back for updates.

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