The stunt was organized by the group Improv Everywhere, a group that says its mission is to cause "scenes of chaos and joy in public places." Participants met up at six locations around the city and got their half-naked marching orders.
"It's a place to meet people that's not your traditional bar scene," said Brady Kirchberg, 26, who was taking part in his third no-pants ride.
Kirchberg was one of about 300 people who gathered at the Great Hill in Central Park and then split up to ride two different trains.
The event's somewhat complicated choreography involved a few people taking off their pants once the subway door closed. At the next stop they got off and waited on the platform for the next train. Meanwhile more people took off their pants, got off and boarded the train behind the first one. After four of five stops that second train was full of half-clothed straphangers.
"I just wanted to do something fun and spontaneous," said Ashley Kemp, 24.
Participants were told to act like nothing unusual was going on. Most folded their pants, stashed them in their backpacks and resumed reading magazines or chatting with their companions.
Like many of the men, Kirchberg wore relatively modest boxers as he leafed through The New Yorker. Some women drew stares in their skimpier undies.
But most bystanders were blase.
"It looks chilly," said Dawn Young, waiting on the platform at 96th Street alongside several pantsless people.
Gintas Norvila said, "It's the first time I've seen it. It looks very interesting," and went back to reading Dostoevsky's "Notes from Underground."
The event started in 2002 with just seven people.
Some of Improv Everywhere's other actions have included "Surprise Wedding Reception" and "Slo-Mo Home Depot," in which scores of "agents" pretended to shop at the home-improvement store and then froze in place.