A man was arrested outside the Trump National Golf Club in California Sunday while allegedly monitoring "security-planning activities" ... just days before Donald Trump was set to headline a fundraiser there.

Jeanine John Taele was allegedly taking pictures and walking the grounds when he was spotted by plainclothes federal agents ... multiple outlets report.

When L.A. County Sheriff's Department deputies arrived, they found a loaded magazine in Taele's pocket. They also found a loaded pistol and more ammo in his car at the golf course parking lot.

On Monday, authorities searched his home and found a ton of weapons ... including an illegally modified AR platform rifle, a .45-caliber pistol, body armor, high-capacity magazines and a even more ammo.

Taele was arrested on suspicion of carrying a concealed firearm and possessing prohibited ammunition.

Trump is scheduled to appear at the golf course Tuesday night for a Republican National Committee fundraising dinner.