As outsider Authentic claimed the crown at the Kentucky Derby, armed supporters of police and anti-racism protesters squared off near the famed Churchill Downs track in the latest violence of America’s long, hot summer.

A large group of protesters marched toward the Churchill Downs track chanting “No Justice, No Derby” – a nod to an earlier call by activists for the historic Louisville race to be cancelled.

It was being held without spectators to prevent the spread of coronavirus.

A battle to the finish! pic.twitter.com/8Z5vA0zwt5

— Kentucky Derby (@KentuckyDerby) September 5, 2020

As preliminary events got underway, about 200 members of NFAC – a militia group which has protested against police killings of black people – gathered at a park just outside and were inspecting their weapons.

Lousville has emerged as one flashpoint in a summer of unrest due to the death of Breonna Taylor, a black 26-year-old killed when police burst into her apartment with a “no-knock” arrest warrant in March.

Earlier on Saturday, some of the counterprotesters outside Churchill Downs, brandishing pistols and long guns, squared off with a group of Black Lives Matter protesters and got into shoving matches.

People on both sides screamed, faces centimetres apart.

After about 45 minutes, police cleared the people from the park but the protests outside Churchill Downs continued.

The counterprotesters included about 250 pro-police demonstrators called The Angry Vikings, who were armed and dressed in tactical gear.

Demonstrations against racism and police brutality have swept the US since May 25 when George Floyd died after a Minneapolis police officer knelt on his neck for nearly nine minutes.

A masked and heavily armed counterprotester confronts BLM activists while a Kentucky local holds a sign pleading for peace. Photo: Twitter

In Portland, Oregon – another flashpoint – police arrested 27 people overnight, mostly on charges of interfering with law enforcement or disorderly conduct.

“Officers began to make targeted arrests and in some cases moved the crowd back and kept them out of the street,” police said.

One arrested protester was injured with a “bleeding abrasion” on her head.

Portland has become the epicentre of demonstrations, with nightly protests over the past three months.

These have at times turned into clashes between demonstrators and officers, as well as between right- and left-wing groups.

Police shot and killed a self-declared anti-fascist activist in Washington state on Thursday night as they moved in to arrest him on suspicion he fatally shot a right-wing counterprotester last weekend in Portland.

The Trump administration deployed federal officers to Portland in July to crack down on the protests.

President Donald Trump signed a memo on Wednesday threatening to cut federal funding to “lawless” cities, including Portland.

Democratic challenger Joe Biden has accused Trump of stoking violence with his rhetoric.

In Rochester, New York on Friday night nearly 1000 demonstrators marched downtown to protest the March death of black man Daniel Prude in police custody.

This week, both Trump and Biden visited Kenosha, the Wisconsincity where protesters, police and militia clashed after a white policeman there shot Jacob Blake in the back multiple times.

–with AAP