Around 3,500 people in New York and thousands more in 48 cities around the world stripped down to their underwear for the tenth annual No Pants Subway Ride.
The annual event, organized by New York-based collective Improv Everywhere, has been creating "scenes of chaos and joy in public places" since its inception in August 2001.
Videos of past Improv Everywhere "missions" have quickly become viral sensations, often generating views in the millions.
Social networking sites have helped boost the popularity of flash mobs (groups of people that suddenly assemble together and perform random or seemly pointless acts in public places before dispersing without necessarily acknowledging other participants), making it easier to coordinate worldwide events and easier for participants to share photos and videos of the event after it takes place.
The official video of Improv Everywhere's No Pants Subway Ride 2011 made its way onto the internet on January 17.
It can be viewed on Improv Everywhere's official YouTube Channel,
Photos and full reports on the event can be found on the Improv Everywhere website, http://improveverywhere.com.
For more flash mob videos, have a look at Listropolis' list of 24 Flash Mobs You Need to See to Believe: