Tony Salazar has come to the beach with a folding chair and a sense of entitlement. “The beach is for everybody,” he says as he picks his spot on a picturesque patch of Malibu shoreline. “This is something that belongs to the public. It doesn’t matter how much money you have.”

He gestures towards a sweep of architect-designed houses on a stretch of sand to his left. “Just because you buy the front house, doesn’t mean you own the beach.”

Salazar should know – he is 52 and in real estate. He says he can understand why the rich and famous residents of this affluent beach city in Los Angeles County would want their privacy. But over the past few years, many of Malibu’s 13,000 residents have been watching with alarm as public access rights to the 27-mile coastline have come under threat.

Many celebrities and multimillionaires own sprawling Malibu homes overlooking the Pacific, including actors Robert Redford and Angelina Jolie, the rapper Dr Dre, the director Rob Reiner and media mogul David Geffen. In an effort to protect their privacy, some homeowners have now taken matters into their own hands by employing security guards to patrol the sands in front of their houses.

Twice in the past few weeks, members of the public have been asked to leave Malibu’s Escondido Beach by a uniformed security guard who wrongly claimed they were on private property and threatened them with a fine for trespassing. The incidents have reignited tensions in the city, which has for years been at the heart of a battle over beach access in California.

“There’s 27 miles of beach in Malibu – it’s one of the few public spaces we have in LA County – and out of those 27, 20 are blocked by private development,” says Jenny Price, an environmental writer and co-creator of a popular app which tells the public how to access specific beaches. “Those 20 miles have for decades been treated as a private riviera … It’s the most egregious example of privatisation of public land in Los Angeles.”

A sign threatens beachgoers with penalties for trespass. Photograph: David McNew/Getty Images

The state’s earliest laws dictate that the area between the water line and the mean high tide line is public land. To put it simply: wet sand equals public beach. In theory, anyone could walk the 1,100 miles of California’s coast and never set foot on private property.

Of late, however, wealthy homeowners have taken to erecting their own “No trespassing” signs and putting out traffic cones to discourage people from parking their cars. The hiring of private security guards is the newest skirmish in a long-running battle.

Noaki Schwartz, a spokesperson for the California Coastal Commission, which oversees development and promotes conservation in the state, tested the waters herself when she set out for Escondido Beach a fortnight ago with her six-year-old daughter and a friend. Within minutes of sitting on the sand, says Schwartz, a uniformed guard with a clipboard walked over.

“He was polite but pretty firm and said I was trespassing and needed to leave and if I didn’t leave, I would be fined $1,500 and probably get a citation for trespassing.”

When Schwartz refused, the guard called two police officers from the local sheriff’s department. Despite handing over her business card and a map which highlighted the relevant access areas, the three of them were forced into an unpleasant confrontation.

“I found it a bit unnerving,” Schwartz says. “I was just a mom with a six-year-old on the beach. I didn’t even have a towel. I was sitting there with my purse. We should have been fine. Honestly, I don’t think people should have to walk around with legal documents to show they can be on a public beach.”

According to Price, the problem stems from the City of Malibu’s disinclination to stand up to homeowners. Price recently sent a folder of photographic evidence to the council detailing illegal no-parking signs along the Malibu coastal roads.

“They said, ‘Oh no, those are on private property. We can’t do anything about them. We consider them requests.’ These are signs that say the vehicle will be towed! … What you’re seeing in Malibu is the wealthy and powerful who have the money to fight [for their interests].”

Geffen’s mansion is notorious locally for having a set of fake garage doors painted on to a patch of wall adjoining the road. When Aaron Crow, a 42-year-old software engineer, parked his car in this spot last month, he was given a $53 ticket for obstructing access – despite there being no official parking restrictions in place and, ultimately, no garage in place for which access was required. He is now contesting the ticket.

But Price says the situation is slowly getting better. The 1976 Coastal Act codified common-law public access and is more frequently enforced now that the public are increasingly aware of what is happening. In July, following a decade-long legal fight, a public walkway opened up giving access to a 1.5-mile stretch of sand known as “Billionaire’s Beach”.

Geffen had spent years fighting against the path, which had been promised by the previous owners in 1983 in exchange for planning permission. Steve Lopez, a columnist for the Los Angeles Times, has even suggested staging a Sand Aid concert to continue the fight for beach access. “It’s about taking back the beach,” Lopez wrote in a recent article, “no matter how expensive the legal fight may be.”

As for Tony Salazar, he has more important things on his mind. The heat is rising. The surfers have just started riding the waves. For the next few hours, he will be soaking up the Californian sunshine – on a patch of beach that is rightfully his to enjoy.

• This article was amended on 13 September 2015 to correct an editing error.