Joan Rivers is "resting comfortably"

"I want to thank everyone for the overwhelming love and support for my mother," Melissa Rivers said in a statement released by Mount Sinai Hospital. "She is resting comfortably and is with our family. We ask that you continue to keep her in your thoughts and prayers."

A law enforcement source told ABC News that Rivers was rushed to Mount Sinai Hospital in New York City after an emergency call that she was in cardiac arrest treatment.

The New York City Fire Department transported a woman matching Rivers' description after receiving a call that the woman was in cardiac arrest at an Upper East Side clinic, Yorkville Endoscopy, an FDNY source said. The law enforcement source confirmed the woman was Rivers.

TMZ reported that Rivers was having surgery on her heart and cardiac treatment at the clinic.

Rivers, 81, had been hard at work of late, co-hosting her E! TV show, "Fashion Police" and doing stand-up comedy. According to Ticketmaster, she had seven shows planned across the U.S. for November.

"Ignore aging: Comedy is the one place it doesn’t matter. It matters in singing because the voice goes. It matters certainly in acting because you’re no longer the sexpot. But in comedy, if you can tell a joke, they will gather around your deathbed," she wrote in the Hollywood Reporter in 2012. "If you’re funny, you’re funny. Isn’t that wonderful?"