"We're seeing break-ins in Mill Creek Park and Rolling Hill Park," said Lt. Michael Keenan.
Police say cars in both lots were hit yesterday within about 90 minutes.
Jill Walters of Narberth said the incidents have become frequent.
"They just root through the glove compartments, and recently they've been happening," she said.
Walters added that she now adjusts her routine when visiting the parks.
"I tend to lock up my car here because this place is known to have people do that," she said.
There have been 21 reports of theft from vehicles in Lower Merion Township in less than a month.
Keenan said groups of thieves often move quickly through neighborhoods.
"Groups of people may concentrate in an area, run up and down the street, and grab as many door handles as they can. If 1,000 are locked and 10 are opened, then you're getting 10 cars' worth of stuff."
"I hope by bringing attention to it, we stop whatever is going on," said Abbie Leibowitz of Wynnewood.
Police are following up on stolen credit cards and other items to determine when and where they're being used, in hopes of identifying suspects.