By Danica Kirka in London

Mind the pants.

Hundreds of Londoners headed down to the Underground train, stripped down to their underwear and travelled around a bit, trying to look as though nothing unusual was going on.

As if.

This was the Official No Trousers Tube Ride, an annual event with no point other than injecting a little levity into the bleak midwinter.

No deep meaning, no bigger motive. The only goal was to be silly, if but for one Sunday afternoon.

“There’s so much bad, so much not fun going on,” said ringleader Dave Selkirk, a 40-year-old personal trainer. “It’s nice to do something just for the sake of it.”

After gathering at the entrance to Chinatown, dozens of clothing anarchists trooped through the icy streets to the Piccadilly Circus Underground station in central London where they boarded their first train.

The only hiccup was that the cars were so crowded some people couldn’t shed their trousers.

Selfies were taken. Grins were exchanged. Tourists looked puzzled.

The first stunt in this vein was held in New York in 2002, the brainchild of local comedian Charlie Todd.

His idea was this: Wouldn’t it be funny if someone walked onto a subway train in the middle of winter wearing hat, gloves, scarf — everything but pants?

The idea took off, and no pants days have been held all over: in Berlin, Prague, Jerusalem, Warsaw and Washington, DC, among other cities.

London hosted its first big reveal in 2009.

Basil Long, a lawyer, showed up at the meeting point in a down coat and hat on a freezing winter afternoon.

But after his journey underground in the warm tunnels of the Tube, he had been transformed, wearing only a white shirt with bold rainbow stripes, pink underwear and Underground-themed socks.

“I just saw it online and I just thought, why not? It’s always a question, isn’t it?” he said. “When someone is asked why they climbed Everest, they were just like, why not?”