PORTLAND, Maine (NEWS CENTER) -- More than a hundred dancers gathered in Portland's Congress Square Park for a Thriller flash mob on Thursday.

While the seemingly-impromptu event seems like all fun and games, it takes a month of preparation.

"My dad did a lot of make up for us. Halloween was one of our favorite holidays and we would get an extensive make up kit and ultimately we were the subjects of the make up, but eventually it became fun to do it." Reggie Groff has spent his life training for this; turning dancers into zombies in a bar around the corner.

"Zombies are fun," Groff said. "They're easy, they're grotesque. It’s really hard to go wrong."

"I’ve always wanted to do a big performance like this, and we started five years ago in Monument Square and we had 30 dancers when we started," said Sutton.

The class has grown to more than 100 dancers; most have no background in this, they just wanted to have some fun.

"I’ve had a lot of people tell me this is a bucket list thing I wanted to do," said Sutton. "My youngest person is 19 and my oldest person doing it is in their 70s, so we have all walks of life."