An "astounding" arsenal of more than 1,000 firearms has been discovered hidden in the Los Angeles mansion of a suspected gun runner.

Key points: A man who lived at the mansion has been arrested over suspicion of unlawful transportation and sale of firearms

A man who lived at the mansion has been arrested over suspicion of unlawful transportation and sale of firearms An LAPD officer says he has never seen so many weapons in his 31-year career

An LAPD officer says he has never seen so many weapons in his 31-year career The arrested man is a longtime companion of real estate mogul Cynthia Beck

It took about 30 police and federal agents 12 hours to remove all the weapons from Girard Saenz's home, which is in the posh Bel Air neighbourhood and worth millions of dollars.

Acting on a tip regarding illegal firearms sales, authorities raided the property and found guns strewn throughout several of the home's rooms.

Photos and video from the scene showed piles of rifles, shotguns, assault weapons and pistols laid out in the driveway.

Others revealed a floor stacked with boxes of ammunition.

The weapons will be itemised and booked into evidence, police Lieutenant Chris Ramirez said.

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"I had never seen so many weapons in my career of 31 years," he said.

"That's such a big arsenal in a residence like this, in this type of neighbourhood. It's astounding."

Following the finding of the weapons cache, Mr Saenz, 57, was arrested on suspicion of unlawful transportation and of giving, lending or selling an assault weapon.

The 57-year-old was later released on a $US50,000 ($71,400) bond.

The federal Bureau of Alcohol, Tobacco, Firearms and Explosives said in a statement that authorities received a tip about a person "conducting illegal firearms transaction outside the scope of the federal firearm license" that the person possessed.

The man arrested over the gun stash is a building contractor. ( ABC News: Supplied )

Possible links to moguls

Mr Saenz is a longtime companion of real estate mogul Cynthia Beck, who owns the property and many others in California, according to records cited in local media.

Ms Beck has three daughters with J Paul Getty's son Gordon Getty.

She bought the house in January 2001, but it remains unclear what, if any, connection she has to the events on Wednesday (local time).

It is unclear whether Ms Beck lives at the home where the weapons were found.

Mr Saenz has been a licensed general building contractor since 1994 and operates a business in San Diego called Gerry Saenz and Associates, according to public records.

He was arrested in Malibu at a home on suspicion of brandishing a firearm in 2017, but the matter was not considered for charges after the alleged victims told deputies they did not want it pursued.

Mr Saenz was released within a few hours of being detained.

ABC/AP