MONTREAL—For the crowd eating outside at Place-des-Arts on Thursday night, secrecy and elegance was the name of the game.

Montreal’s Diner en Blanc saw 4,200 meet at an outdoor picnic organized completely by social media. The location of the event was announced only 30 minutes before the mob was scheduled to meet.

Everyone who gathered at Place-des-Arts was garbed head-to-toe in white and toted their own food. When the event was over, attendees left with their belongings, leftovers and garbage, leaving the site as it was upon arrival.

Being green is one of the cornerstones of the celebration of white.

The impromptu flash mob event was created in Paris in 1988 by Francois Pasquier, with the goal of sharing a delicious meal with friends and respecting public spaces, along with a touch of elegance and secrecy.

His son, Aymeric Pasquier, originally from Paris now hails from Montreal. Sandy Safi runs the worldwide event out of Montreal, which held its fourth tres-chic picnic on Thursday.

"I think it's definitely a new generation of events, especially the fact we're creating a worldwide network with it with people who have the same love of the event," Safi said, adding that the current "foodie culture" is also contributing to its popularity.

They co-produced a Diner en Blanc in New York City last year. Invitations were so coveted that there were 31,000 on the waiting list, but the Battery Park picnic was limited to 1,200 people.

Safi said when they began getting requests to hold events in other cities, "we decided to create a network where it would allow people in each city to create their own event using the same values and guidelines and history of what was set up originally but applying it to their city."

They advise organizers keep the event "small" in the first year, limiting numbers to 1,300 people.

Place du Canada in downtown Montreal was the setting for the second gathering in 2010, which consisted of 3,200 people.

"It's the kind of space I passed in front of 150 times and never actually gave it attention," Safi said. "It was just like passing through a park which I'm sure you do every day. Now suddenly you can't look at it the same way ever again."

The event takes place in 20 cities worldwide, including Toronto, Vancouver and Quebec City.

--with files from The Canadian Press.