The Lewis star, who is married to Billie Piper, told his Twitter followers, "Park in loading bay to run into chemist to get medicine for sick child. Eldest son asks to stay in the car.
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"I agree as I will be minutes. Return to car to find police community support office telling me my son has been screaming in the car.
"He looks fine to me. I ask him if he is ok and he says he's fine. I ask him if he was screaming, he says 'no'.
"Get in the car. As I close the door… office calls me a 'disgusting and appalling human being' and infers my son is lying."
Laurence, 35, said he would be making a formal complaint about the officer.
"I cannot believe it," he said. "I've got their number. Looking forward to writing to whoever it is you write to when you are harassed by the police."
Legally it is an offence to leave any child under the age of 16 alone for any period of time — even briefly in a locked car.
Prosecution would depend on the circumstances, however, and law experts say that in each case a parents needs to take into account the age and maturity of the child and the length of time.