Image copyright Getty Images Image caption A protester lobs a brick at police during protests in Washington during the inauguration of Donald Trump

In a divided America, two groups at the extreme ends of the political spectrum are doing battle online, and on the streets.

The alt-right - a disparate group of pro-Donald Trump provocateurs who critics say are bigoted white nationalists - has a reputation for trolling and online bullying. Now some believe they may have met their match in the form of a group of left-wing anarchists whose tactics are arguably more extreme.

They're called "antifa", short for "anti-fascist". The movement has its roots in 1930s Europe, but has had a low profile for much of the intervening period. Now the recent surge in nationalist movements across the globe has given it a new enemy to fight.

Antifa activists say they are committed to fighting fascism and racism in all its forms. Some aren't averse to violence, and the movement wasted little time in making its presence felt. Protests held during Donald Trump's inauguration turned violent. Restaurant windows were smashed, a car was set on fire and objects were thrown at the police. More than 200 arrests were made.

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Image copyright Getty Images Image caption A Trump campaign hat set on fire by protesters during demonstrations in Washington

But the video which went viral that day wasn't of the rioters; it was one that featured the white nationalist Richard Spencer being punched by a masked man. Almost immediately mocking memes flooded the internet, including a number of videos of the attack set to music.

Far from condemning the attack, many antifa activists revelled in it.

"Every time anyone replays that video, 11 million ghosts rejoice along with them," an anonymous activist who runs an antifa Reddit group told BBC Trending. The 11 million figure, they say, refers to the victims of fascist regimes through the ages. "We as a society are so unwilling to condone Neo-Nazi philosophies ... that the video has become a part of the popular zeitgeist is a beautiful thing."

Not surprisingly, the fact that an act of violence has been turned into a propaganda coup infuriated many on the alt-right, amongst them Chuck Johnson, an influential figure in the movement.

"We've certainly reached a very tribal point in the culture where people cheer on violence," he told Trending. "Richard is not my favourite person on the right, but you should be able to give an interview on the street without being assaulted.

"I thought that was pretty disturbing to say the least."

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Last week the alt-right got a measure of revenge when Johnson published, on his website, the names, dates of birth and addresses of the 223 people who've been charged in connection with the Washington protests.

In internet speak, this is called "doxxing" - publishing someone's details without their permission, potentially laying them open to the threat of being harassed by anyone with a personal or ideological grudge against them.

It's a tactic used both by the alt-right and antifa. Johnson himself is perhaps most famous for publishing the home addresses of New York Times reporters and trying to reveal the personal information of a woman who was subject of a retracted Rolling Stone article about an alleged campus rape. He runs another site which crowdsources "bounty" rewards for actions against liberals. Some of the rewards are offered for revealing personal information.

Johnson defended the doxxing of the Washington protesters to BBC Trending.

"I don't have an issue with accused criminals having their addresses published," he says. "I don't think it's a problem."

The antifa activist whom we spoke to was equally unapologetic.

"Antifascists absolutely do engage in doxxing active members of hate groups." the anonymous activist said. "To ensure the safety of those who they would victimise from the shadows, we must bring them into the light."

At the same time, they don't like doxxing - when it happens to them.

"Many of those arrested in DC had absolutely no connection to any illegal action," the activist claimed. "Now, they face the threat of harassment by the most hate filled elements of society."

Online, there's a constant cat-and-mouse game. On alt-right and antifa message boards there's waves of trolling, spies, and constant rumours about infiltration. But the fight is also happening on the streets. In addition to the Washington protests, in recent weeks there have been a number of incidents in which both sides say they have been targeted for attack solely on the basis of their political beliefs.

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