While we theoretically have a code for such a thing, if it's come to that point, it's rarely used. I have been on some all-county calls where you might end up with a couple dozen officers from five or six agencies at one scene.
By far the most frightening call I have ever heard (I did not respond as I believe I was tied up) was a dispatcher keying up to advise that they were getting 911 calls of an officer being assaulted at a particular fairly busy intersection. Nobody had called out there, so they only had the calls. A 29 year veteran supervisor went out with a drunken idiot student who had walked out in traffic. This guy wasn't trying to jam the kid up, and the mere fact that he deemed him worth stopping tells me how egregiously stupid and careless the attention drawing behavior must have been. The kid ended up behind the officer with his arm around the officer's neck, pulling up and leveraging with the hood of the car. The officer lost consciousness, but a bystander's threat to call the police scared him off.
Everybody made it home, nobody was seriously hurt, and the perpetrator was eventually located, arrested, and convicted, but it was an absolutely surreal thing to hear broadcast like that.
"Logic is rarely the engine that propels a police department forward."
-David Simon in "Homicide"