A white man who held a gun as he yelled racial slurs at a group of black teenagers during a confrontation at a Martin Luther King Day demonstration in Miami is now facing hate crime charges, prosecutors say.

Miami-Dade State Attorney Katherine Fernandez Rundle announced Tuesday that in addition to an initial charge of carrying a concealed firearm, Mark Bartlett is now facing counts of aggravated assault with prejudice, improper exhibition of a firearm and carrying a concealed firearm -- all with hate crime enhancements that could result in stiffer penalties if convicted.

Bystander cellphone video shows Bartlett carrying a handgun and yelling racial slurs at the group of teens who were on bicycles and blocking traffic January 21 during an affordable housing demonstration, CBS Miami reported. The demonstration reportedly coincided with a larger "Wheels Up, Guns Down" event, during which protesters ride motorcycles and all-terrain vehicles at high speeds in traffic, popping wheelies and riding while standing on the seats, according to The Associated Press.

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Cellphone video of the confrontation on Brickell Avenue Bridge starts with Bartlett's girlfriend, Dana Scalione, yelling "you just ran over my foot." Things then escalate, showing Bartlett walking toward the teens and holding the gun. Scalione at one point calls the teens "thugs" but was not charged with any crime, The Associated Press reported.

Bartlett and his attorneys did not appear in court for an arraignment Wednesday but entered a written plea of not guilty, the Miami Herald reported. Bartlett has apologized for using the racial slurs and said he was protecting his girlfriend.

"Mark went to protect Dana and extract her from the mob surrounding and taunting her," Bartlett's attorney Jayne Weintraub said, the paper reported. "It would not have mattered if these people were red, white or blue. This was not a hate crime."

The courtroom was reportedly packed with people supporting the teens. A teen who was involved in the incident, 16-year-old Jamel Anderson, told CBS Miami he feels the incident was racially motivated.

"He was just mad," said Anderson. "I think, to me, he just don't like black people."

Fernandez Rundle said her office filed the additional charges after completing interviews with final witnesses on Friday.

Mark Bartlett CBS Miami

"Actions matter and words matter," Fernandez Rundle told reporters. "Crime matters and the law matters. The charges are about words coupled with actions. We are looking at the totality of the evidence and what the witnesses and the victims are saying and that is why we felt these were the appropriate charges."

Lee Merritt, an attorney representing six of the teens involved, lauded the charging decision. Merritt had pushed for hate crime charges in the case and has filed a civil rights lawsuit against Bartlett and his girlfriend, reported CBS Miami.

"It means a lot to these families of the children who are victims of what we have always believed was a hate crime," Merritt told CBS Miami. "We hope this leads to a vigorous prosecution that will lead to the appropriate sentencing. Hate crimes are on the rise in the past few years and to deter them we have to use the laws that are on the books."

Attorneys for Bartlett, however, said the state attorney "succumbed to political pressure" in filings the charges.

"Clearly this mob of people who were commandeering traffic, and taunting passengers, while wearing masks and gloves, were not peacefully protesting -they were not peacefully doing anything. They were committing multiple crimes for which the State Attorney is not holding them accountable," the lawyers, Jayne Weintraub and Jonathan Etra, said in a statement released to CBS Miami. "....This charging decision is a disgraceful miscarriage of justice, and we intend to remedy it – by vindicating our clients in a court of law; not in the court of public opinion."

Under the hate crime enhancements, Bartlett faces a possibility of five to 55 years behind bars, the station reported. Prosecutors will ask for an increase in his bond during a February 26 hearing, for which Bartlett was ordered to appear.