WATCH: 'Thriller' flash mob breaks out in downtown Portland, Maine

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PORTLAND, Maine (NBC News) - Halloween came a little early in Portland, Maine.

A flash mob invaded downtown by dancing to Michael Jackson's "Thriller."

The walking dead swarmed Monument Square and put on a scary show for passersby.

Organizers say participation is open to anyone, and they start meeting to learn the choreography about three weeks in advance.

The "Thriller" flash mob has become an annual event since 2013.