You have a First Amendment right to record police officers serving as civil servants in public officially. This means that the officer does not have the right to delete your records without a court order. The policemen can’t take the phone away or demand to see the tapes. They can only interfere with the recording if they are prevented from doing their job.You must not violate the law during filming, including trespassing on private property, and obstructing law enforcement. It’s best to film with a non-secret recording device as the law may become more nebulous in some states where there may be an expectation of privacy. The problem is, that some cops don’t know this.