South Yorkshire Police said there has been an increase in thefts from vehicles left unattended in the town.
PC Jack Goodall, of the Doncaster Central Safer Neighbourhood Team, said: “Today I have attended an incident in a car park in the town centre of Doncaster where a male has made his way into a car, and whilst in the car, taken some property. “There has been a rise in the number of thefts from motor vehicles. Offenders will walk around public car parks and try car door handles for cars left unlocked.
“Please make sure your cars are properly locked before leaving them in a public car park. And when parking at night, park underneath lighting and in view of CCTV where possible."
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He added: “There are a number of offenders in the town centre who are deliberately trying car door handles.
“We are doing our best to locate and prosecute these people, but we can avoid property being taken and anyone being a victim of crime if we take extra care to make sure our cars a properly locked.”
Anyone with information on the culprits should call South Yorkshire Police on 101.