Law enforcement sources tell TMZ ... there are surveillance cameras all up and down the section of Hollywood Blvd. where Ian got into it with the bikers, as well as cell phone footage of the incident, so investigators have a lot to work with here.

Our sources say this group of bikers bumped Ian's car and when he got out of the vehicle, things got heated and the attack ensued ... which backs up what Ian's claiming.

Cops tell us this is not an isolated incident and it's happened before with large groups of motorcyclists, and bicyclists too.

Police recommend anyone finding themselves in Ian's shoes should stay in their car, call 911 and remain calm ... because mob mentality can escalate very quickly in these situations.

The way cops see it ... it's better to let insurance handle things as long as it's just property damage and it's not worth getting into it with a large group of people.

One angle from the attack shows bikers using their helmets to beat the crap out of Ian's car ... damaging his windshield while his 10-year-old daughter was inside.

Our sources say the case will most likely be turned over to the Los Angeles County District Attorney because it's a case of felony vandalism with over $1,000 in damage.

As for the battery aspect of the case, we're told any criminal charges there would likely be misdemeanors, because there were no major injuries.

Our sources say LAPD is aware of an Instagram post one of the bikers allegedly published on social media ... and they're looking into it.