A "No Kings Day" protest erupted into violence when a group of young Trump supporters were embroiled in a fight with demonstrators."

An onlooker captured a video of a Trump supporter throwing punches at an elderly man in Vero Beach, Florida, at around 1.30pm on Saturday.

He was not expecting to encounter a 3,100-strong crowd, gathered to oppose President Trump.

"We noticed a group of about five young males with MAGA hats and flags," he said. "

"As people with a history in law enforcement and military, we stopped because we noticed there was a different vibe coming from these kids."

The witness said the young men were "purposely putting themselves in the center of groups of people," which raised red flags. "

What started as back-and-forth chanting quickly escalated when violence broke out.

"The young kid, from what I heard, someone touched his hat, at which point he started throwing punches," the witness said. "

"He started continually punching an older gentleman until the police came over and broke it up."

The incident particularly frustrated the witness, who said the protest had been remarkably peaceful up to that point.

"We had just walked that entire protest and even said as people in law enforcement, it was kind of nice to see that it was pretty peaceful," he said."

But the police response left much to be desired, according to the witness.

The bystander said the officer appeared biased, "accusing the No Kings protesters of surrounding the kids" when in reality "the kids put themselves in the center of the group."

No charges were filed because police couldn't locate the elderly victim who was assaulted, the onlooker claims.