Should I call 911?

It was a quiet residential street in Kew Garden Hills/Flushing, there would not be many walking traffic normally.

If you saw more than 20 people gathering on the street, yelling, pushing each other, would you call 911?

After one or two minutes, cars stopped and they still didn't move. I didn't want bad thing happened before it was too late, so I called 911.

After few minutes they left liked nothing happened.

But that is not the end of my story. The worst thing happened was no police ever showed up. After the Crowd left, I was still waiting for the police for more than half an hour. I thought may be I need to brief the police what happened. That made me think of the Central Park Sexual Assault in 2000 that the police didn't stop the crime right in front of them.