The activists arrived outside the Wi Spa in Westlake Saturday morning, some prepared for the worst.

Several wore bike helmets and vests with extra padding. N.W.A.’s “F— Tha Police” and Kendrick Lamar’s “Alright” filled the air.

They were met by far-right extremists who over the last few weeks had turned a debate over transgender access at a Korean spa into a rallying cry.

It didn’t take long for the dueling protests to dissolve into disarray.

By the end, the LAPD had used projectiles and batons and arrested 40 people — mostly for failure to disperse. Several said they sustained injuries at the hands of police.

“I knew it was going to be violent. I didn’t know it was going to end up like that,” said Jessica Rogers, a 31-year-old who had come to document the event and support transgender rights and was among those arrested.

In the aftermath, the police are facing questions about whether officers used excessive force. But the incident has also exposed the power of a viral video given widespread attention in the right-wing press and social media.

The spa has become the latest hot spot for clashes between far-right groups and the left in L.A., kicked off by the video taken by an irate customer in late June.

The video showed a woman arguing with Wi Spa employees after she said she had seen a customer with a penis in an area that is reserved for women. The Wilshire Boulevard facility has some gender-separated areas with changing rooms and Jacuzzis.

The footage was quickly amplified by an international network of right-wing activists, pundits and media outlets, including Breitbart, the Gateway Pundit, RealClearPolitics and TheBlaze, a publication founded by Glenn Beck. Message boards where anti-trans activists gather, including Mumsnet, saw thousands of comments.

The spa told The Times Monday that they are required to follow California law that prohibits businesses from discriminating against customers based on race, gender, sexual identity or expression.

“Like many other metropolitan areas, Los Angeles contains a transgender population, some of whom enjoy visiting a spa,” it said in a statement, adding that the spa “strives to meet the needs and safety of all of its customers.”

Brian Levin, director of the nonpartisan Center for the Study of Hate and Extremism at Cal State San Bernardino, said the protests are indicative of a “democratization of hate” that has allowed far-right groups with a variety of ideologies to come together around flash-point events after they are amplified online and in conservative media.

“The lightning rod stuff that occurs on social media … then gets slung like a catapult by influencers, and then ends up as violence in the streets,” he said.