police

after u telling him to shift the van due to customers not coming in ur shop, and he doesnt do so, then u can file a complaint in the nearest police station about the bad conduct

that parking of police vehicle was a temporary event 

or

its a regular parking spot for the police vehicle?

Based on the answer you can proceed further. if temporary, then forget and move on...

if permanent then approach higher authorities with written complaint along with proofs

It would be appropriate to contact, consult and engage a local prudent lawyer for analysing facts/ documents, professional guidance and necessary proceeding.

There is no cause to be agitated unless the parking is a regular feature. Certain things in life are required to be taken in their stride.

please search the number and address of SP office in your area and there is tweeter account of every SP office of state....district level or collector office ......do the simple thing tweet to DG office...SP Office and Collector of District .......

and find a press reporter in your area and escalate the matter.....