Stand at a safe distance and use your phone to record a video of what is happening. Do not try to interfere with what the police are doing or stand close enough to obstruct their movements.
You have the right to observe and record plainly visible events in public spaces. Don’t try to hide the fact that you are recording. Police, when performing their jobs, are not entitled to a reasonable expectation of privacy. (The people police are interacting with may have privacy rights that would require you to notify them of the recording.)