What is the largest Guinness World Record event ever held, and how many people participated?

The largest Guinness World Record event ever held was the attempt to break the record for the most people performing 'Thriller' dance at the same time. The event was organized by the city of Mexico City, Mexico, and took place on August 29, 2009.

Over 13,000 people participated in the event, which took place in the city's main square, the Zocalo. The participants, who ranged in age from 8 to 80, rehearsed for several weeks leading up to the event, and were led by professional dancers.

The attempt to break the record was successful, with 13,597 people performing the dance simultaneously, breaking the previous record of 242 people.

The event was not only a celebration of the Guinness World Record, but also a tribute to Michael Jackson, who had passed away just two months earlier. The participants wore Michael Jackson-style clothing, and the event featured a giant screen showing footage of the singer's most famous performances.