6:19 PM PST -- We're told an official report has been taken, and it lists Ian as the victim. Police have not yet made any arrests and are still investigating the incident.

5:33 PM PST -- Law enforcement sources tell TMZ they're aware of the incident and speaking with the parties involved. No word yet on if there will be any arrests.

Ian Ziering got jumped by a bunch of bikers running amok in Los Angeles -- a horrific moment caught on camera ... and one helluva lousy way to end his year.

The 'Beverly Hills, 90210' star fell into the crosshairs of several assailants Sunday who were cruising down Hollywood Blvd. on a variety of motorized mini bikes and weaving in and out of traffic ... including, it seems, Ian's vehicle -- which we're told may have been hit.

You can see Ian hop out of his car after one of the bikers seemingly catches his attention, possibly from a collision ... and the altercation erupts in a violent brawl, all seen on video obtained by TMZ.

Check it out ... Ian starts wailing on the guy in question, but soon -- he gets ganged up on by the others ... who begin to whoop on him mercilessly as he tries to make a run for it. Like we said, the cilp is absolutely bananas ... especially considering this a well-known actor.

We're told this beatdown ended with Ian getting back in his car and driving off ... ditto for the bikers, who got the hell out of there. It doesn't appear cops were called -- so you could say it was left in the streets. Not to make light, but this pretty is 'Sharknado'-coded, no?

We've reached out to his camp for comment ... so far, no word back. Here's hoping he's alright -- and if not, someone get Fin Shepard on the horn, STAT, to start kicking ass!