Lunada Bay, an out-of-the-way cove on the Palos Verdes peninsula south of Los Angeles, is one of the best big-wave surfing spots in California. In the shelter of its steep, semi-circular cliffs are a pristine beach and the promise of 20-foot winter swells. And, unlike so many other prime breaks on the US west coast, its waves are miraculously uncrowded.

That’s because anyone who wants to surf there must first get past the Lunada Bay Boys, an infamous local surfing clique known routinely to chase off outsiders with threats, harassment and, in some cases, violence. At the top of the dirt path that leads down to the beach, the Bay Boys once posted a sign that warned: “Unlocals will be hassled.”

This week, though, it is the Bay Boys who are under threat, after they were targeted by “unlocal” surfers with a class-action lawsuit, which seeks to classify the group as a criminal street gang. If successful, the suit would see the Bay Boys banned from gathering at their coveted surf spot, and ordered to pay fines and compensation to the plaintiffs.

Unwanted visitors to Lunada Bay claim to have been pelted with rocks and had their tires slashed. According to the lawsuit, the gang’s members “dangerously disregard surfing rules when it comes to visitors… run over visitors with their surfboards, push visitors, hit visitors, slap visitors, harass visitors by circling them, and hold visitors underwater.”

Contrary to the image of a typical LA gang, the Bay Boys are largely white and affluent, like their Lunada Bay neighbours. Among the 100 or more members of the group are firemen and school-teachers, said Vic Otten, the lawyer who filed the suit. “What’s really shocking,” he added, “is that in many cases these are men in their 50s and 60s.”

Among the plaintiffs in the case are a police officer from nearby El Segundo, who says he was hassled by the Bay Boys while trying to surf at Lunada Bay last year, and Diana Reed, a filmmaker who claims to have been sexually harassed while visiting the beach in January. The gang members are “certainly obnoxious,” said Mr Otten, “but some are also dangerous.”

One Bay Boy “shook up a can of beer and sprayed Reed and her camera with it, and poured beer on Reed’s arm,” the lawsuit alleges. “They told her they thought she was ‘sexy,’ and filmed her while they told her she ‘excited them’… [A Bay Boy] briefly exposed himself to Reed while he was changing into his wetsuit.”

Since local media first reported the lawsuit earlier this week, Mr Otten said he had received more than a dozen calls per day from other potential plaintiffs. “These are people who’ve been harassed over the course of 20 years,” he said. “They’re upstanding citizens who are angry and who want to join the lawsuit.”

In fact, the allegations of intimidation stretch back several decades. Local activist Geoff Hagins, now 60, said he was first attacked by locals at Lunada Bay in 1969, when he was a teenager. “I went there to surf and got caught on the side of the cliff for 20 minutes while they threw rocks the size of softballs at me,” he said.

In 1995, when his 10-year-old nephew was threatened by the Bay Boys, Mr Hagins went to Lunada Bay with a local news crew, causing an altercation that was caught on camera. He won a $15,000 out-of-court settlement from one of the gang members and a restraining order that banned the group from threatening outsiders. Clearly, it didn’t last.

Surf culture may have a reputation for being mellow, but it also contains a strain of aggressive, so-called “localism”: surfers often protect their native breaks from interlopers to prevent them becoming overcrowded. Another notorious group, the Silver Strand Locals, are known to patrol their beach in Oxnard, 70 miles up the coast from Palos Verdes.

The Oxnard punk band Aggression encapsulated the Silver Strand Locals’ mindset with its 2003 track “Locals Only”, which features lyrics such as: “Going into my waters and what do I see / Out-of-town goons in front of me / All I wanna know is what they’re doing on my waves / Gonna have to show them how we behave.”

The Bay Boys rule their fiefdom from an open-air clubhouse on the rocks above the beach, a stone and wood structure built, needless to say, without official permits. From this stronghold, they reportedly drink and hurl abuse at visitors or each other. “Most of them aren’t even good surfers,” Mr Hagins said. “They spend most of their time drinking.”

Recently, the California Coastal Commission demanded local officials crack down on the gang for restricting access to what is supposedly a public beach, and urged them to either approve the clubhouse as a public space, or tear it down. The Palos Verdes authorities have been accused of spinelessness and even complicity in their attitude to the Bay Boys.

That is reflected in the lawsuit, which names police chief Jeff Kepley as a defendant, and asks a judge to order police to investigate the gang’s alleged crimes. “Palos Verdes Estates has a long history of deliberate indifference in not investigating or otherwise policing acts of violence and vandalism against visiting beachgoers,” the suit claims.

After he took charge of the Palos Verdes Estates police department last year, Chief Kepley promised to step up patrols at Lunada Bay. But in an interview with radio station KPCC, he said the Bay Boys’ activities were simply not violent enough to prosecute. “There's not shooting and stabbings and things that you typically associate with street gangs,” he said.

One unnamed Bay Boy addressed the lawsuit this week, in an interview with the surfing website Surfline.com. “If you’re down there and we don’t know you, then someone or multiple people are going to say something to you and at the very least make your whole session completely miserable,” the man said.

He added, however, that he was “not aware of anyone being physically assaulted [at Lunada Bay] in years… The reason why nobody has been arrested at the bay is because there hasn’t been an incident that has warranted an arrest.”

The lawsuit initially named eight members of the Bay Boys, including four men from two generations of the same family. Mr Otten said more defendants would soon be added to the suit, but described the eight in question as “the worst of the worst”. Several of the men named in the suit reportedly have criminal records.

One is linked to the recent violent robbery of a local off-licence. Another, 27-year-old Michael Papayans, pleaded not guilty last month to an assault at an LA Dodgers baseball game. His grandmother, Sheila Papayans, told local paper the Daily Breeze that the lawsuit was ridiculous. “Most of these people have been to college. They all have good jobs. [Surfing] is their hobby, just like fishing and swimming,” she said. “They are not a gang.”