Hundreds of supporters of President Trump descended on Washington, D.C., on Tuesday, planning to protest Wednesday's expected congressional certification of Democrat Joe Biden’s victory in the Nov. 3 presidential election.

As of late Tuesday, police had already arrested at least six people for a variety of offenses, including weapons and ammunition possession, possessing a stun gun and assaulting a police officer, authorities said.

Many of the protesters gathered in Freedom Plaza, east of the White House, to cheer on Trump's claims of election fraud, a day before the congressional vote on the election results. Hundreds remained in the plaza into the night amid cold temperatures and as rain swept into the area. 

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People attend a rally at Freedom Plaza Tuesday, Jan. 5, 2021, in Washington, in support of President Donald Trump. (Associated Press)

"I’m just here to support the president," said David Wideman, a 45-year-old firefighter who traveled from Memphis, Tenn.

Trump shared his support for the protesters in a Twitter message on Tuesday. 

"Washington is being inundated with people who don’t want to see an election victory stolen by emboldened Radical Left Democrats. Our Country has had enough, they won’t take it anymore! We hear you (and love you) from the Oval Office. MAKE AMERICA GREAT AGAIN!" he wrote. 

Protesters were later seen on social media clashing with law enforcement. 

Local authorities had braced for the possibility of violent clashes ahead of the rallies. Businesses boarded up their windows in the downtown area in fear of damage to their stores after many were impacted during the unrest and rioting in the nation's capital last year. 

Mayor Muriel Bowser called up the D.C. National Guard to help with crowd and traffic control ahead of the planned demonstrations this week. She urged residents to stay away and avoid confrontations. 

"We will not allow people to incite violence, intimidate our residents or cause destruction in our city," Bowser said. 

Bowser reminded the public and anyone attending the rally that firearms were prohibited "within 1,000 feet of any First Amendment activity." 

The mayor's statement comes as police have been posting signs near where the rally was to take place, notifying the public that all firearms would be prohibited. Police said the prohibition on carrying weapons at the rally includes city residents who have a concealed carry permit.

Trump said he would be speaking to his supporters in Washington at 11 a.m. Wednesday during a "SAVE AMERICA RALLY" on the Ellipse, just south of the White House.

He also urged Antifa activists to stay out of Washington -- using the term "Terrorist Organization," in a separate tweet on Tuesday.

"Law enforcement is watching you very closely!" he continued. 

Organizers planned to rally into the night on Tuesday and again all day on Wednesday on the Ellipse. An afternoon march was also planned to the U.S. Capitol, where Congress will be voting on whether to affirm the election results.

A number of prominent Trump supporters were expected to attend, including Texas Attorney General Ken Paxton and longtime Trump ally Roger Stone, the recipient of a pardon by the president.

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Earlier this week, DC police Chief Robert Contee III said guns were not allowed at the demonstrations and anyone in violation of D.C. law would be arrested.

During previous pro-Trump protests, police sealed off Black Lives Matter Plaza, just north of the White House, but the confrontations spilled out to the surrounding streets. Black Lives Matter Plaza was sealed off again Tuesday.

"We know that historically over the last few demonstrations that BLM Plaza has been a focal point," Contee said a day earlier. "We want to make sure that that is not an issue."

Fox News' Bradford Betz and The Associated Press contributed to this report