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10:30 AM PT -- Randolph allegedly had roughly 2 POUNDS of weed on him at the time of the arrest. We're told the weed was in a "large backpack."

That's a lot of weed.

8:02 AM PT -- TMZ Sports shot Randolph leaving jail moments ago ... with a big smile on his face!

Randolph didn't seem concerned one bit and casually walked to a waiting ride.

NBA star Zach Randolph was arrested in L.A. on Wednesday night -- igniting a crazy disturbance that required cops to call in the cavalry for backup, TMZ Sports has learned.

LAPD officers were called to the Nickerson Gardens area in L.A. around 10 PM -- and when they arrived, cops found a large crowd of people smoking, playing loud music and blocking the roadway.

Law enforcement sources tell us 3 people were seen grabbing their waistbands and running -- so cops went after them.

Three people were detained -- including Randolph, who was eventually arrested for possession of marijuana with intent to sell, A FELONY. We're told the "intent to sell" part was due to the large amount of weed found.

Another man who was arrested was an ex-con in possession of a gun, law enforcement sources say.

After the arrests, we're told things got crazy -- the crowd became unruly and LAPD had to call for backup.

We're told 6 cop cars were vandalized -- tires slashed and windows busted -- and cops formed a battle line to restore order.

In the end, cops recovered 2 guns, drugs, a significant amount of cash and 2 vehicles were impounded -- though it's unclear if any of those things belonged to Randolph.

Randolph is listed at 6'9", 260 on his booking sheet, which also says bail was set at $20,000.

FYI, Randolph spent years with the Memphis Grizzlies before signing a brand new 2-year, $24 million contract with the Sacramento Kings.

We've reached out to Zach's rep for comment -- so far, no word back.

Story developing ...