Police say social media posts are promoting more illegal meetups in Nassau this weekend.

"They're doing it for one reason and one reason only, to cause unrest in our communities," said Nassau Police Commissioner Patrick Ryder.

On Thursday, more than 100 teenagers gathered in the parking lot of the Green Acres Mall in Valley Stream.

Commissioner Ryder says word of a so-called "takeover" spread on social media.

Eleven teens ranging in age from 13 to 19 years old were arrested on a variety of charges, including riot and robbery.

"You want to be able to go to the mall with your family, your wife, your kids, be able to do some shopping, have some food just do something in peace," said Craig Caparelli, of Lynbrook.

The commissioner says an Instagram post is promoting another meetup at the Roosevelt Field Mall this Sunday with the caption "Pull up. Bring boxing gloves."

Police are onsite at every mall in Nassau ready to respond.

"When we find this guy that is posting these things, we're going to arrest him and then we're going to hit him with a big fat bill. He is going to pay for the expenses for the resources we had to put out there last night and again today," Ryder said.

In Suffolk, County Executive Ed Romaine says police are on the lookout.

"We have an intelligence unit of the police department that monitors 24 hours a day social media, and we are prepared; we have increased patrols in our malls," said Romaine.

Commissioner Ryder says all 11 people arrested in Valley Stream were New York City residents.

He says police are on standby, and shoppers should feel safe this weekend.