Police arrested a 20-year-old man after he allegedly rammed his car into a crowd of anti-racist protesters marching against a white supremacist rally in the US state of Virginia on Saturday.

The assailant was identified as James Alex Fields Jr and was charged with second-degree murder, one count of hit-and-run, failure to stop with injury and three counts of malicious wounding, according to local media reports.

The incident in Charlottesville killed 32-year-old Heather Heyer, an anti-racist protester, and injured dozens more.

The alleged attack took place during a march against Saturday's Unite the Right rally, one of the largest white supremacist gatherings in recent US history.

What happened?

Fields' grey Dodge Challenger reportedly slammed into a crowd as hundreds of anti-racist and anti-fascist marchers made their way through downtown Charlottesville.

I just saw multiple bodies fly over car windshields. The black car that did it drove off. This is horror. This is a war zone #DefendCville — Kim Kelly (@GrimKim) August 12, 2017

Video footage of the incident shows bodies flying over cars and others being hurled to the ground by the force of the impact.

The car reversed, hitting more people, and then drove off.

Who is James Alex Fields, Jr?

Fields' arrest was announced shortly after the incident. Local news outlets obtained his booking photo and posted it on Twitter.

JUST IN: Booking photo of James Alex Fields, Jr. @NBC29 pic.twitter.com/9nxtsvqNmt — Henry Graff (@HenryGraff) August 13, 2017

Photos captured from the scene showed that the car's licence plate was registered to Fields, who is a resident of Maumee, Ohio, according to multiple news outlets.

The Federal Bureau of Investigation's Richmond Division, the Civil Rights Division and the Attorney General's Office for the Western District of Virginia are investigating whether it was a civil rights incident.

The Toledo, Ohio-based daily newspaper The Blade spoke to Fields' mother, Samantha Bloom, after his arrest was announced.

Bloom said that her son went to Charlottesville to attend the Unite the Right rally. "I told him to be careful," his mother said. "[And] if they're going to rally to make sure he's doing it peacefully."

Speaking to the Associated Press, she said: "I thought it had something to do with [US President Donald] Trump. Trump’s not a white supremacist."

"He had an African-American friend so ...," she said before trailing off.

According to public records published by BuzzFeed, Fields is registered as member of Trump's Republican Party.

Social media posts

Several news outlets and social media users captured screenshots of posts from a Facebook account believed to have belonged to Fields. It is neither deactivated nor deleted.

Many of the posts express support for the alt-right, a loosely knit coalition of far-right groups that includes white supremacists and neo-Nazis.

Photos from the now-defunct fb page of the James Fields charged with committing murder today in Charlottesville. He's clearly alt-right. pic.twitter.com/eNQBZfbsq2 — Alex Press (@alexnpress) August 13, 2017

Others showed support for Syrian President Bashar al-Assad, whom alt-rightists and other far-right activists have publicly stood with in recent months.

A picture posted on the Facebook page of James Fields who ran his car through the crowd in Virginia today. pic.twitter.com/xGB4vclsFG — Yashar Ali (@yashar) August 13, 2017

The account also regularly posted photos that expressed support for Nazism, fascism, racism and President Trump.

White supremacist group

The Southern Poverty Law Center, an Alabama-based watchdog that monitors hate groups, and social media users quickly pointed out that Fields had been photographed marching with Vanguard America, a white supremacist group that is part of the alt-right.

A photo of Fields marching with apparent Vanguard America members have been circulated on the internet. In the image, he is carrying a shield with the group's emblem.

Looks like killer member of "Vanguard America," neo-Nazi group part of "Nationalist Front" led by Matthew Heimbach. Known for vandalism. pic.twitter.com/pjseWbThrN — It's Going Down (@IGD_News) August 13, 2017

Vanguard America has been active in white supremacist recruitment efforts on university campuses across the country this year.

After the images went viral online, the organisation denied that he was a member. In a statement posted on Twitter, Vanguard America said that Fields was "in no way" affiliated with the group and that "the shields were freely handed out to anyone who was in attendance".