My understanding is you have to identify yourself,as in, state your name and where you are from. I don’t think you have to show any credentials because we don’t have laws requiring us to carry papers.
Assuming this went down as described, I'm going to split from the majority here and say this looks like a legitimate complaint to me.
In California if you are not operating a motor vehicle you are not required to produce or have ID. Cops should have done a quick field interview and left them alone. (They are also not legally required to cooperate. But two minutes of simple conversation and that’s that. )
Hiibel v. Sixth Judicial District Court of Nevada, 542 U.S. 177 (2004), held that statutes requiring suspects to disclose their names during police investigations did not violate the Fourth Amendment if the statute first required reasonable and articulable suspicion of criminal involvement. Under the rubric of Terry v. Ohio, 392 U.S. 1 (1968), the minimal intrusion on a suspect’s privacy, and the legitimate need of law enforcement officers to quickly dispel suspicion that an individual is engaged in criminal activity, justified requiring a suspect to disclose his or her name.
The Court also held that the identification requirement did not violate Hiibel’s Fifth Amendment rights because he had no reasonable belief that his name would be used to incriminate him; however, the Court left open the possibility that Fifth Amendment privilege might apply in a situation where there was a reasonable belief that giving a name could be incriminating.
http://en.wikipedia.org/wiki/Hiibel_v._Sixth_Judicial_District_Court_of_Nevada