Video shot during the event captured the moment tensions between the two groups boiled over when a group of men who apparently attended the rally approached a group of counter-protesters dressed in black.
As the two groups approached a ditch, a fight broke out, with people scuffling on the ground, punching each other, and spewing obscenities.
Some were seen using objects, including one man who used an American flag pole to strike another person.
It is unclear what provoked this encounter and which group the counter-protesters were associated with.
The altercation lasted less than 2 minutes before it began to slow.
When cops arrived on the scene most of the men had fled, however at least three people were arrested following the brawl.
"We respect everyone's right to peacefully assemble," said Fort Collins Police Services Chief Jeff Swoboda.
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He continued: “Truly supporting a cause means representing its values. Committing crimes in our community is not a way to support police.
"We stand for safety, period. We want all of our community members to feel safe here in Fort Collins and we reject any form of hate.”
Following the death of in May of this year, hundreds of protests have taken place across the US - demanding justice and an end to police brutality.
died after being arrested by Minneapolis police outside a grocery shop in .
Footage of the arrest on 25 May shows a white police officer, kneeling on Floyd's neck for almost 9 minutes.
A medical examiner on June 1 classified Floyd’s death as a homicide, saying his heart stopped as police restrained him and compressed his neck.
The cause of death was listed as “cardiopulmonary arrest complicating law enforcement subdual, restraint and neck compression,” according to the official information from the Hennepin County Medical Examiner’s Office.