APD says several of their officers were forced to respond to dozens of demonstrators Sunday -- who they say departed a music festival to descend on a construction site in a wooded area of neighboring Dekalb County, and they unleashed mayhem when they arrived.

The construction site is part of a massive, 85-acre police academy/training facility that city officials have been trying to build for years -- and which is already partially in use.

A lot of people in Atlanta have opposed its presence and have protested on and off for about a year or so -- one protester was even shot and killed earlier this year. Things had simmered down since then ... until Sunday evening.

Check out this video Atlanta PD released, which they say shows some of the attack they were under ... it's pretty insane. You see people dressed in black throwing items at officers as they scramble to close gates.

Cops say these protesters -- who they paint as agitators -- were throwing Molotov cocktails, setting off fireworks and chucking rocks and other projectiles in officers' direction. Soon enough, fires did start breaking out on some of the police equipment ... utterly wild scene.

Eventually, enough backup arrived to drive the protesters away ... and at that point, cops spread out into the surrounding woods to look for stragglers. APD says they detained upward of 35 individuals, but unclear if any of them were arrested or if they'll be charged.