Air Carrier landing SAN received taxi instructions from an Airport Operations vehicle causing confusion; the reporter expressing concern regarding the Airport Operations intervention.

Narrative

We were landing in SAN on Runway 27. The sun was setting and was just at the horizon; about 25-30 degrees off of the runway center line to the left. As we exited the runway; we were planning to taxi directly from the turn off into the alleyway for {gate}. As we made the left turn off of the runway; the sun was directly in our faces. I advised the Ground Controller that we were clear of Runway 27 and wanted clearance to {gate}. The Controller said; 'Taxi B to {gate}.' At that time a RJ had started moving in our direction on Taxiway B and was blocking access to the taxi center line. The sun was in our faces; and I could not even see that there was an airplane there until we had moved toward the alleyway to {gate}. The Captain moved to the left to enter the ramp and alleyway in compliance with ATC instructions. Suddenly; an airport Ground Operations vehicle came on the frequency and told us that we could not proceed and that we had to make a 180 degree turn and taxi to a remote area until the traffic had cleared. Ground Control told us to make the 180 degree turn; but the Captain told the Controller that we had room to enter the alleyway and could do so safely. The Controller told us to proceed. The Ground Operations Person again came on frequency and told us that we had to remain on the taxi center line. I replied that we were unable and that the taxiway was blocked by the RJ. We taxied to the gate without further incident. The airport Ground Operations Person came to our gate and spoke to the Captain about the incident. He stated that we had done what we were supposed to; but the RJ was wrong in moving before receiving a taxi clearance. This Ground Operations Person should not have access to transmit on ATC frequencies and should not be attempting to direct traffic. That is the sole purview of trained; accredited ATC personnel. If that Ground Operations Person has an issue; it should be brought to the proper personnel after the event and not over the radio. This person has no authority to direct traffic; and he made the situation worse and more confusing by injecting himself into it.